Adds profitbricks driver

This patch adds support for the [Profitbricks IaaS](http://www.profitbricks.com).

The changes I made to the Instances and StorageVolumes collections were necessary because the Profitbricks IaaS has two physical Datacenters (EU and US) in which one can create as many Realms as one likes. Due to this specialty Images are bound to the physical location of the datacenter and Storages are bound to the virtual Realm.

While testing the driver via the web interface I encountered a few other small issues (mostly unassigned variables), which relate to the other changes.

Regards,

Dominik