Small refactoring, mostly related to design-related code review comments...

Small refactoring, mostly related to design-related code review comments.

  - no intermediary "client" anymore. Base class for interface definition, and
    UltraDNS implementation derived from that base class.
  - Removed "get_records_by_type" method, added "rtype" param to the "get_records" method instead.
  - Removed docstrings from UltraDNSClient class, because interface is defined in a base class.
Just FYI - we use the following module name convention for our clients:
`services.py` - we put any clients there, actually everything that interacts with web services (so in that architecture your UltraDNSClient should live there)
`models.py` - we put all business logic there, everything we want to be done on client side (to work with data from web services like with models)