[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
`Element.iter` is not available in Python 2.6 (https://docs.python.org/2/library/xml.etree.elementtree.html#xml.etree.ElementTree.Element.iter), so please use `findall` instead (`datacenter.findall('.//*')`).