Fixes to the align branch

Some small bug fixes for the align branch and changed variable names to PEP-8 style variable names.

Fixed the gale church implementation so the included doctests pass & used log probabilities in the distance function. 
The implementation details has changed somewhat from @tmarek's original implementation as I couldn't get the same distance scores as the ones from the reference implementation in the the Gale-Church Aligner paper.

cf https://github.com/nltk/nltk/issues/559