Python 3 support

Hi!

I've implemented Python 3 support for python-ecdsa. It supports Python 2 and 3 from a single source.

It uses six (embedded copy in the tree) to remain compatible with 2.x (particularly 2.5) and 3.x. All the tests pass in 2.5, 2.6, and 2.7, but this should probably undergo a more complete inspection by someone who has worked with this more extensively.