Add provisioning for Google Compute Engine

Generating the Vagrantfile works like charm. Not all options from [vagrant-google](https://github.com/mitchellh/vagrant-google) are supported in the UI. Only this options needed for basic provisioning.

And there is only the Debian Wheezy 7 x64 Backports with PHP 5.5 as image available, cause this the only one I tested the provisioning with. Needs more testers for the other images and PHP versions.

Oh, and anyone can make a nice logo for the GCE button? I'm not the perfect designer ^^

closes #457 :)