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.
They key difference is that we don't need to know that there is an UltraDNS webservice. It's functionality is isolated, and the only code which knows about this is YoUltraDNS code. Some day UltraDNS can easily switch to some binary protocol (actually, that what they were using before switching to REST), and this won't impact Yola code at all, except YoUltraDNS.