[Libcloud-589] ProfitBricks Compute Driver Support

I have completed support for the ProfitBricks compute driver. This adds support for the core compute driver functions against ProfitBricks. The JIRA ticket can be found here:

https://issues.apache.org/jira/browse/LIBCLOUD-589

Let me know if there are any issues or if it can be accepted or not.

Thanks. 
-matt
The method should take `volume_id` and not `volume` (and instance of the `Volume` class) argument.

The user should already have all the volume information if it has a reference to the `volume` object. Unless of course, the Volume class is instantiated manually with just an id, but that's an invalid use of the API in this case.