Adds support for hiding "future tasks"

When parsing tasks, look for the "t" (aka threshold) tag. If the
threshold date is in the future, then the task is a "future task". A new
preference controls the global (un)hiding of such tasks.
Changing the default size of QTodoTxt
Refacto: applying "source format" from pydev
Fixes #18: adding a filtering widget
Implements some visual improvements in the task HTMLizer.
Fixed several outdated doctests

Fixed several doctests, updating the expected result with the newer task
format. Fixed some doctests that were using an uppercase 'X' as a "task
complete" indicator. Fixed the test runner so it now accepts absolute
paths in module names. Added some tests artifacts to the .gitignore
file.
Fixes #29: We can now edit a task with Enter_key
Merge branch 'feature/task-htmlizer-improvements' of https://github.com/chaghi/QTodoTxt into chaghi-feature/task-htmlizer-improvements

Conflicts:
	qtodotxt/lib/task_htmlizer.py
On Mac Os X, there is no font called "mono" so PySide will fallback to
another font which was not monospaced.

So now I load a monospaced font (Ubuntu Mono) at the begining and use it
if the font "mono" is not found.
Clean unit tests
Fixes #30 : Merge branch 'feature/hide-future-tasks' of
https://github.com/chaghi/QTodoTxt

I just add a Filter in order to stay consistent with others part of
QTodo