2to3 available - passes tests with py25-27,py31-32,pypy

What the title says.

A subtle difference that may affect Python 2 users: In Python 3, `json.loads()` accepts (unicode) `str` only, and some serializers pack objects into only `bytes` (e.g. `pickle`) .  So `itsdangerous` tries to negotiate with the string type serializers are using; and in that case, `latin1` encoding is assumed.