Manage pythons with pyenv

This module (forked but changes submitted upstream) installs pyenv and can install the latest versions of Python 2.6, 2.7, 3.1, 3.2, 3.3

It's got a bit of a weakness at the moment in that you can only install one version of python3 (e.g. 3.3.3) - as soon as you install the second version (e.g. 3.2.5), it will break the build of the subsequent versions.

That's because the build does a `which python3` and if that fails falls back to `python`. However if you have pyenv running and a python3 installed, the `python3` shim exists and returns a helpful message about not existing in that python version, rather than interpreting python. I'm working on this, but it doesn't stop the module being useful.

/cc @jabley @tombooth @alexmuller @bradleywright