Rewritten tag library

Uses modern batteries:

* twitter instead of python-twitter
* django-classy-tags instead of django-templatetag-sugar
Rewritten testsuite

Uses httpretty and sure to be agnostic of used twitter lib and any API calls.
More clean and readable code.
Separated fixtures in json files.
New version of docs, reflecting changes in lib

Added Diederik van der Boor as contributor