fixed name error in compile_regexp_to_noncapturing(), cf https://github.com/nltk/nltk/issues/580
removed doctest from compile_regexp_to_noncapturing as it doesn't work anymore
added the 'flags' argument to sre_compile.compile()
Merge pull request #582 from jskda/pull_request.regexp_noncapturing

Pull request.regexp noncapturing