Change return types of API calls

[Ntr0]

Currently the models include fields that are not actually part of the api models, especially the Headers.
Since Headers sometimes contain viable information as the location header for tracking a request's status, all api calls should return model, response and error. So if a header is needed, it can be extracted from the response. Example:

datacenter, response, err := client.CreateDatacenter(myDc)
location := response.Header().Get("location")

This would be a breaking change, so please include this for the next version