Also add reference to where the code came from.

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.

1. Tests for relative months and years are now executed against fixed
   test dates.
2. Add additional test for "special cases".
3. Add test for leap years.

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

- Increment months strictly: 01.01.2015 + 1m = 01.02.2015.
- If there are fewer days in target month, date is rounded down:
  31.10.2015 + 1m = 30.11.2015.
- Years are incremented as 12 months.
- Leap years are 366 days long: 01.03.2015 + 1y = 01.03.2016 (not
  29.02.2016 as it would have been if year was equal to 365d)

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.