[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
Instead of using `**kwargs`, please explicitly declare all the available arguments[[1]] and prefix them the ones which are not part of the base API with `ex_`.

E.g.

```python
def create_node(self, name, image, size=None, location=None, ex_availability_zone=None, ex_internet_access=True, ex_ram=None, ex_cores=None, ex_disk=None)
```

As you can see, I also used `location` instead of `datacenter` (this refers to the comment about NodeLocation I've mentioned above).

[1]: https://libcloud.readthedocs.org/en/latest/development.html#don-t-abuse-kwargs