sorting dictionary text by decreasing word frequency. useful when analysing dictionary quality
Merge branch 'develop' of https://github.com/renaud/gensim into develop

Conflicts:
	gensim/corpora/dictionary.py
added sorting-by-frequency as an option = a new method parameter
using iteritems(dict) for Python3 compatibility
Merge remote-tracking branch 'renaud/develop' into develop

Conflicts:
	gensim/corpora/dictionary.py
add test for Dictionary.sort_by_word