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 not related with whoosh itself (see context [here](https://bitbucket.org/mchaput/whoosh/issue/324/highlighting-under-python-3-fails) ), but to how we were calling the ContextFormatter. This is explained in detail on the commits, if anyone is interested.