Added python support

Hello. I'm quite a fan of what you've done here as I'm also a fan of Vagrant and Heroku.

However, my weapons of choice are Python and Django and I found the default build was missing some parts. So, I've tried to help fix that.

Specifically, I've added a few bits to the postinstall script to include python setup and dev tools as well as virtualenv and a rather minimal configuration to make all of that work out of the box for python devs.

I've also included Foreman to make the VM work even more like Heroku, in that it respects a project's Procfile and .env configuration

I've done a little bit of testing locally, it seems to work pretty well out of the box. I hope this will make the VM even that much better.