Support custom fields

[travispavek]

[travispavek]

From PR #32 Run object: One method needs to be refactored, and I wasn't sure what the proper approach should be. The method is structured like so:

 @property
 def custom_status_count(self):
     return self._content.get('custom_status_count')

However, the the actual run objects returned from the API look like this:

{ <...> "custom_status1_count": 0, "custom_status2_count": 0, "custom_status3_count": 0, "custom_status4_count": 0, "custom_status5_count": 0, "custom_status6_count": 0, "custom_status7_count": 0, <...> },