Add a more Pythonic wrapper to make repeater access easier

[philpem]

The current repeater interface requires fairly intimate knowledge of the Hytera radio protocols. This is a disadvantage, because it can make code quite hard to read.

It would be nice to have a more Pythonic interface which simplifies the interface and may include:

• Connection management
• Wrapper functions for RRS messages to avoid having to instantiate packets
• Event driven functionality and callbacks for events (connect, disconnect, receive message/broadcast)
• Event filtering/selective subscription (listen only for certain types of message)

[philpem]

• Functions to send requests and wait for the appropriate response -- e.g. Button Request -> Button Reply.
  • Should include blocking (wait for reply), non-blocking callback (callback is called on reply) and non-blocking event (event is broadcast on reply) response patterns