v2 API, python 3 support.

Closes https://github.com/jplana/python-etcd/issues/3

Also, adds support for python 3.3 (or above).

I changed a little bit the etcd.EtcdResult type given how directories are treated now; also, we now have 3 basic methods: write() read() and delete(), from which you can derive any other action. I left the old methods around so that people would not need to rewrite their code from scratch when migrating.

I know this is a quite-large patch, but it's all due to my personal needs evolving during the development.