use sre to process regexes properly to remove capturing groups
fix to https://github.com/nltk/nltk/issues/545, renamed convert_regexp_to_nongrouping() to compile_regexp_to_nongrouping() as there isn't an official python module to stringify a parsed regex
Merge branch 'develop' into pull_request.regex_remove_groups

Conflicts:
	nltk/internals.py
fixed some bugs in convert_regexp_to_nongrouping_parsed()
raise ValueError when there are backrefs in regexes
Merge pull request #574 from jskda/pull_request.regex_remove_groups

Regex remove groups bug