[rackspace|compute] Fix typo in attachments model.
[rackspace|compute] Handle malformed API responses.

When deleting a Cloud Network, the API returns a plain text
response even when the Content-Type is set to
application/json. In the event of a JSON parsing error, this
change sets the request body to an empty hash.
[rackspace|networks] Add Cloud Networks requests.
[rackspace|networks] Add networks model and collection.