Restore backwards compatibility with the blob.json property

Adds a to_json method that takes same arguments as json.dumps
Also fix a problem displaying a repr for a blob when the text is a
non-ascii character
Add WordList append and extend methods
Merge branch 'master' into dev
Python 2 __repr__ and __str__ compatiblity fix
Versino 0.5.0 -> 0.5.1