sorted() was run on a list of Todo objects, which may produce different
results. Instead, sort on the item's number (i.e. its index in the
TodoList).

This should fix the occasional failing of test_do_subtasks2.

Feature is not ready yet, but since it's a part of issue #2 I already went
ahead and pushed the first part.

This can be used for ordinal tag filters, for instance for getting the
todo items with a date older than 6 months:

topydo ls 'due:<-6m'

Fixes a bug that ListCommand wouldn't show items matching key:2014-12-12

Also added testcases to handle the combination with strict recurrence
well.

Conflicts:
	test/FilterTest.py

Recorded with ttyrec, processed with tty2gif [1], turned into animated
GIF with ImageMagick and then optimized using ezgif.com [2]

TTY recording also included with this commit.

[1] https://bitbucket.org/antocuni/tty2gif
[2] http://ezgif.com/optimize/8f097380ed.gif

Conflicts:
	test/FilterTest.py

Handle every identifier type in a separate function and try to obtain a
result from one of them.

Conflicts:
	test/DoCommandTest.py
	test/SortCommandTest.py
	test/TodoListTest.py
	topydo/lib/TodoList.py
	topydo/lib/TodoListBase.py