v0.3.0

BUG FIXES
===========
Fixed clear() method which didn't clear element frequencies.
Fixed pop() method which would remove n-grams which are a prefix of the
popped n-gram if they had no other children.
Fixed repr() method which was equal to str() and now gives an
evaluatable string.

NEW FEATURES
===========
Added equality check.
Added initialization data in constructor.
Added methods to get different n-gram sizes and elements as well as
gives the frequency of given n-gram sizes and elements.

BROKEN COMPATIBILITY
===========
elements() returns set of elements instead of dictionary.
elements() method replaced with ngram_eles().
ngrams_with_eles() method replaced with ngrams_with_all_eles().
sized_ngrams_with_eles() method replaced with
sized_ngrams_with_all_eles().
ngrams_by_pattern() method replaced with ngrams_by_template().