Test ignore_weekends clause in Importance

Also introduce freezegun.freeze_time decorator.

Update tests for Importance

2 more tests of calculation importance from due date.

This increased (branch) coverage to 100%. A few corner cases were
missing with strange default commands in the configuration.

Coveralls doesn't support branch coverage, while codecov does.

A couple more tests

Thanks to @mruwek for reporting.

The comment is not stripped from the value by the ConfigParser, so

    identifiers = text ; foo

didn't work.

Although the documention says that inline comments are allowed:
https://docs.python.org/2/library/configparser.html?highlight=comment

For some weird reason, the aliases code block was not properly formatted on Github.

Online coverage reports via coveralls.io

"Modules should have short, all-lowercase names. Underscores can be used in
the module name if it improves readability."

Fix unicode values in aliases config in python2

Apparently shlex and ConfigParser.read() don't work well with Unicode
input in python2.

Fix help in prompt mode

`help` is not a real subcommand and without arguments it is of NoneType,
so we need to properly handle it.

 topydo ls -s project

to print all todo items grouped by project.

Properly split alias options wrapped in quotes

It shows the first todo item to be done, optionally with a filter
applied:

topydo next +Chores @Home

So we still have the tags in the master branch.

Thanks to Travis CI and the unit tests, the master branch is almost
always of sufficient quality to be used.

Other pros:

* Wiki content matches the default branch.
* Project stats at Github are up to date.
* Less merge overhead at release time.

This is achieved by importing some modules only when applicable. For
instance, 'ls' doesn't need ChangeSet and also doesn't always need
icalendar. Sorting and archiving is also only applicable to writing
commands (del, add, etc.).

ChangeSet._trim may call ChangeSet.delete() several times, which incurs
a disk write every time. This creates a lot of overhead, especially when
lots of revisions are removed.

Instead, leave the responsibility to the caller of _trim to make sure
that the new content is written to disk. In this case, ChangeSet.save()
the only caller, already writes to disk.

There was a restriction in place that a file should contain the same
number of todo items before and after an edit. This is now removed.

Reason for change is that I lost a lot of edits to a long list of items
and removing one single item from the list. The editor exits and as a
result topydo refused to process it, losing all the changes.

Another reason is that we have `revert` now. In case you severely mess
up, you can always revert.

This reuses the logic that the backup functionality uses to determine
whether a command is read-only or not. In that case, prompt mode doesn't
trigger a warning for ls* commands when the backing todo file was
changed.