Library does not work with Python3 <= 3.2

The library currently relies on two unicode-related things that are not available with Python 3.0, 3.1 or 3.2:

* [Passing ASCII-only Unicode strings to b64decode](https://docs.python.org/3.3/library/base64.html#index-0)
* [Explicit unicode literals](http://legacy.python.org/dev/peps/pep-0414/)

The attached patches should fix the problems while remaining compatible with Python 2.6, 2.7, 3.3 and 3.4.