Python 3 support

Hi!

I've implemented Python 3 support for python-ecdsa. It supports Python 2 and 3 from a single source.

This fixes #3 

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.