Given a todo number, it prints the dependency tree where the given todo
item is involved (it shows its parents and children).

self.todolist.children() may return todo items outside the p_todos list,
so prevent that.

This generates a graph of todo items, where the edges denote
dependencies between items. The tooltip shows the description (visible
in an SVG by using a decent browser).

Based on the idea that I implemented in topydo's predecessor:
Dot output by todo.txt-tools [1]. There, I implemented it as a
subsubcommand of `dep` to visualize dependencies. This implementation,
however, is part of the `ls` command, where you can use a filter
expression to limit the size of the graph.

The todo.txt-tools implementation also prints a box with a lookup table
of the numbers to descriptions. This is lacking at the moment.

To visualize the todo items of a project, run:

    topydo ls -x -f dot +Project | dot -Tsvg -o Project1.svg

(-x is suggested because parents with unfinished todo items are not
printed by default).

[1] https://github.com/bram85/todo.txt-tools/blob/2f9b2baa6382346c5cd9d2272c61ee0e342e25b6/actions/dep#L162

This corresponds to the change made in commit 33794435.

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