[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
Base API method doesn't include `node` argument.

Is it possible to retrieve the id of the node the volume is attached to by retrieving information for a single volume from the API? 

This way we can follow the standard API (`def detach_volume(self, volume)`) and don't need to change the method signature (`def detach_volume(self, volume, ex_node)`).