Python v0.5.0

Changes:

• BertWordPieceTokenizer now cleans up some tokenization artifacts while decoding (cf #145)
• ByteLevelBPETokenizer now has dropout (thanks @colinclement with #149)
• Added a new Strip normalizer
• do_lowercase has been changed to lowercase for consistency between the different tokenizers. (Especially ByteLevelBPETokenizer and CharBPETokenizer)
• Expose __len__ on Encoding (cf #139)
• Improved padding performances.

Fixes:

• #145: Decoding was buggy on BertWordPieceTokenizer.
• #152: Some documentation and examples were still using the old BPETokenizer