fixed bugs in stanford tokenizer doctests; skip doctests if the jar doesn't exist
moved setup_module to bottom of code, see kmike's comments here: https://github.com/nltk/nltk/pull/587/files#r9105156
changed docstrings to raw docstrings for better readability
changed single to double quotes
Merge pull request #587 from jskda/pull_request.577

Fixes to #577