[POC] Behat

This PR executes documentation steps from `installation.rst` with [Behat](http://behat.org).

It makes use of the [shinx-behat](https://github.com/dantleech/sphinx-behat) Sphinx extension which generates behat `.feature` files from annotations within the documentation.

Behat will install SuluCMF in a `_workspace` directory and execute the console, based on the installation instructions.