Wrote the simpler command handlers in each subsystem. For each subsystem, added the Command ID codes as subclasses of IntEnum.
Refactored some names to make them shorter/nicer.

+ send_message() -> tx_message()
+ receive_message() -> rx_message()
+ MsgType -> Message
+ Subsystem -> Subsys
Implemented `echo()` and renamed `*_message()` to `*_mesg()` for brevity.
Implemented `aux.py`. Also updated some comments, documentation, and error messages.
Merge pull request #4 from dwtj/master

Implemented `aux.py` and some of the simpler command handlers.