[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
This method doesn't actually do anything different if a timeout occurs (`None` is implicitly returned in both cases).

On success, we should probably return object in question (latest reference to the `datacenter` object in this case) and on time out, we should throw a timeout exception.

Same goes for `_wait_for_storage_volume_state`.