Vagrant box with tox

To ease local development it's nice to have a VM with all dependencies installed.
Tox is used to create multiple virtualenvs for each tripe of versions/settings:
python-django-settings.

How to use:

* install vagrant (http://www.vagrantup.com/downloads)
* cd in sorl-thumbnail folder
* $ vagrant up
* $ vagrant ssh
* $ runtests

Runtests is an alias to customized tox invocation (some rsync hackery is needed to make it work, but nothing special). More on tox: http://tox.readthedocs.org/en/latest/

E.g. runtests -e py33-1.6-pil works for tests in specific env only.

If PR is ok, I'll add proper docs in next PR.