Fixes whoosh highlight.

These 5 commits fix the whoosh highlight making the test suite to pass on both Python2.7 and python3.3.

The highlight was already broken on Python2.7, and the error in Python3.3 was just a consequence of that.

@toastdriven: the issue was related to how we were calling the ContextFormatter. This is explained in detail on the commits, if you or anyone else is interested.