Python v0.6.0

Changes:

• Big improvements in speed for BPE (Both training and tokenization) (#165)

Fixes:

• Some default tokens were missing from BertWordPieceTokenizer (cf #160)
• There was a bug in ByteLevel PreTokenizer that caused offsets to be wrong if a char got split up
  in multiple bytes. (cf #156)
• The longest_first truncation strategy had a bug (#174)