Priority Aa or AB is (still) not allowed.

Conflicts:
	README.md

The previous commit had the archive condition on the wrong place,
resulting that todo.txt files were not written out at all if archiving
was disabled.

Also introduce the (overall) -a flag to disable auto archiving. -a comes
from the original todo.txt CLI.

It would have been nicer to have DoCommand support the -a flag, however,
DoCommand doesn't know anything about archiving and I would like to keep
it that way.

Addresses issue #42.

(Note that commenting/omitting archive_filename will still by default
write the archived entries to done.txt).

This addresses issue #42.

Conflicts:
	topydo/lib/DCommand.py

Expression-mode for del, depri, do, pri, postpone.

Fell into the trap of calling a function as a default parameter. Because
of this, relative dates used with the 'add' or 'do' command (recurrence)
were based on an offset when topydo was started, not when the subcommand
was executed. Usually these two events occur in the same second and this
bug was unnoticed for a long time. But now with prompt mode, this bug
may surface if you leave running topydo for longer than a day.

- add test for direct todo.txt editing
- remove unneceseary `u()`

- execute_multi_specific() -> _execute_multi_specific()
- execute_not_multi() -> _execute_not_multi()

MultiCommand._catch_todo_errors() prevents occuring of those.

This required some refactoring in MultiCommand and ExpressionCommand.

Whole part about processing flags was refactored. Code related to flags
from DCommand was moved into MultiCommand and reused from there in all
"multi" commands.

New attribute: ExpressionCommand.last_argument. Should be set to True if
command is using the last argument as something other than todo ID or
part of an expression (as in case of postpone or pri). By default set to
False.

New method: MultiCommand.execute_not_multi(): wrapper for functionality
not connected with todo IDs (like `topydo edit` or `topydo edit -d`).

New attribute: MultiCommand.multi_mode: Should be set to True if user
wants to operate on multiple todo items. True by default.

New method: MultiCommand.get_todos_from_expr(): gets todo IDs based on
expression and puts it in MultiCommand.todos.

EditCommand._open_in_editor() now accepts only one argument - file to
editing. It handles now also OSError exceptions. It is used also to
opening todo.txt and done.txt.

MultiCommand.get_todos() doesn't accept anymore todo IDs as an argument.
Instead it's using inherited Expression.last_argument` to decide upon
where to find todo IDs.

Now desired result of DCommand.get_flags() should be more apparent.

Now every call to editor is done by `EditCommand._open_in_editor()` and
whole process of getting editor name is also done there.

When a subcommand finishes, _post_excecute() is called for sorting,
archiving and writing the file. But sorting is also a subcommand, which
in turn also calls _post_execute(), etc, etc.

So let the caller of _execute() decide when to call _post_execute.

This fixes issue #46.

* Make TodoListTester test TodoListBase, make
  TodoListCleanDependencyTester test TodoList. This required moving a
  test case from the former class to the latter.
* Add an additional test for a non-existent todo.

Just make sure that proper arguments are passed to GrepFilter in the
first place.

This started as a refactoring of the View class: to get rid of the
pretty_print() function and anything releated to printers. So the View
class now solely focusses on sorting and viewing a bunch of todos. It's
up to the users of the View class to retrieve the todos in the view and
pass them on to a Printer instance.

The View class was quite involved with the proper Unicode support that
was added lately. There were still encoded string passed through this
class rather than Unicode strings. Getting rid of this affected
basically all other places in the codebase.

The result is that there are less string conversions; printed todo items
stay Unicode as long as possible, until they're actually written out to
standard output or to a file. I got rid of one text_type() call from the
six library, all @python_2_unicode_compatible decorators are gone and
the tests no longer need the utf8() function because everything is
Unicode now.

Remove utf8 function.

No need to toggle the highlighting of projects and contexts anymore. If
you want to turn it off, simply assign an empty value to the
project/context colors:

  [colorscheme]
  project_color =
  context_color =

Thanks to Jacek for pointing out.