[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
Minor thing - it would be nice if `timeout`, `wait_time` and `interval` variables would be declared in the method signature with the same default values which are currently used.

Same goes for `_wait_for_storage_volume_state`