Substantial rewrite of the Porter Stemmer.
This was to:
1) Make it threadsafe
2) Make it more Pythonic and remove at least some of the weird C-like code from
   when it was originally ported from C
3) Remove nonsense comments referring to things like null terminated strings and
   char arrays that are meaningless in a Python context (again the result of it
   originally being ported from C)
4) Make it faster.

A test on the complete text of the King James Bible indicates that post-changes
the stemmer has entirely consistent results with the previous version and is
roughly 40% faster. It is also now threadsafe since nothing is stored in
any instance variables any more; instead things that used to be instance
variables are now passed between functions as arguments.
Merge branch 'master' of https://github.com/nltk/nltk
made change to allow python 2.6-compatibility
frozenset instead of set for vowels
adopted efficiency improvements from https://github.com/nltk/nltk/pull/303