Minor bugfixes
Merge branch 'master' of https://github.com/nltk/nltk
Implemented span_tokenize() method for PunktWordTokenizer.
Merge pull request #517 from jskda/master

Use with ... open() where appropriate instead of just open()
Added doctest for span_tokenize in docstring of the PunktWordTokenizer class.
Merge pull request #520 from huderlem/master

Implemented span_tokenize() method for PunktWordTokenizer.
Fixed bug in path of grammar files; fixed relative import statements
Merge branch 'master' of https://github.com/nltk/nltk
Fixed some bugs in paths and strings.
wrap map.keys() to permit indexing
don't add PY3 if it is already present in the path
Merge branch 'master' of https://github.com/nltk/nltk
link to new version of book
Don't overwrite sys.meta_path in nltk.compat
Merge pull request #524 from pickl-it/meta_path

Don't overwrite sys.meta_path in nltk.compat
replace ngram functions with their iterator counterparts, cf https://github.com/nltk/nltk/issues/527