[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
It seems like we could reduce a couple of lines of code and make this a bit easier to maintain by using a dict and a for loop.

Something along the lines of:

```python
attribute_to_extra_name_map = {
    'ramHotPlug': 'memory_hot_pluggable',
    ...
}

for attribute_name, extra_name in attribute_to_extra_name_map.items():
    if ET.iselement(node.find(attribute_name)):
        value = node.find(attribute_name).text
    else:
        value = None

    extra[extra_name] = value
````