Added support for Django 1.6

- The situations in which contribute_to_class is called have changed.
- Fixed DeprecationWarning about simplejson
- Explicitly set the TEST_RUNNER to the pre 1.6 one.
- Set default=False on BooleanField
Merge.
Keep django version pinned
Better error propagation when querying for tweets.
Added --force option to poll_twitter command which will query for tweets on all queries.
Catch and show twitter query errors in management command, and allow continuing.
pep8 fix.