use runtests.py
Add elementtree_indent function, needed by ch11 (was in nltk_contrib/toolbox)
corrected function name in recursive call
de-anonymised _DrtParser and _LogicParser
renamed parse_ methods to read_ or load_, resolves issue 656
standardised environment variable pathnames to third party software
Replaced FreqDist.keys() with Counter.most_common()
  (keys no longer sorted by decreasing frequency)
Left ConditionalFreqDist.conditions() as is
  (even though these are no longer sorted alphabetically)

Cf nltk/nltk_book/#65
Merge branch 'develop' into twitter