Fix computing start and end indices of sentences
Fix bug with importing textblob with local nltk installed

By adding python_2_unicode_compatible decorator to text/compat.py
Refactor: move all sentence tokenization logic to SentenceTokenizer

Also test pypy build on TravisCI
Bump version 0.5.1 -> 0.5.2
Merge branch 'dev' (-> 0.5.2)