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
Distinction between weigthed and unweighted WindowDiff
Correction of bugs in the proposed version
Merge remote-tracking branch 'DavidDoukhan/master'
Merge branch 'master' of https://github.com/nltk/nltk
incorporated David Doukhan's pull request https://github.com/nltk/nltk/pull/345 , specified float return type, added doctests, misc formatting fixes
updated tests for the length-normalized unweighted version of windowdiff
Correction of the Pk metric

The first implementation I did was 100% wrong.
According to [1] Pk should provide a penalty of about 0.5 to all degenerated segmentations (empty, full, randomized), which is not statisfied by my last implementation.
It was corrected using the explantations provided in [2]

Sorry about that

[1] Beeferman D., Berger A., Lafferty J. (1999)
Statistical Models for Text Segmentation
Machine Learning, 34, 177-210

[2] Unbiased discourse segmentation evaluation
John Niekrasz and Johanna D. Moore
Spoken Language Technology Workshop (SLT), 2010 IEEE
Page(s): 43 - 48
better tests
Merge pull request #353 from DavidDoukhan/master

Correction on the Pk segmentation metric
tweaks for python 3 compatibility
adopted efficiency improvements from https://github.com/nltk/nltk/pull/303
removed module-internal tests for Porter stemmer
reverted changes to porter stemmer, because they inadvertently introduced different behaviour of the stemmer
updated for 3.0a1
updated print function
update windowdiff tests