[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
Please declare all the "exported" / "public" classes using `__all__`.

E.g.

```python
__all__ = [
    'ProfitBricksNetworkInterface`,
    'ProfitBricksNodeDriver`,
   ....
]
```