improve word_tokenize speed

Runs roughly twice as fast on my test set of 40000 sentences. According
to kernprof (http://pythonhosted.org/line_profiler/), the culprits of the
previously bad performance were

    r"([^' ])('ll|'re|'ve|n't|) "

and

    r"([^' ])('LL|'RE|'VE|N'T|) "

which were matching all words due to the | at the end.

Also removed some useless whitespace handling that was being undone by the
str.split just before return.

The contractions handling is now the slowest part @~20% of the time.
Merge pull request #434 from larsmans/faster-word_tokenize

improve word_tokenize speed