- 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.

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

If that configuration file lacks options, the defaults will be used.

This will be used to make sure that tests always run with a fixed,
well-defined configuration, not influenced by how the user happened to
configure topydo.