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
Merge branch 'master' of https://github.com/Copper-Head/nltk