Merge pull request #4 from asr1/master

Added the transmission queue (txq), initial error handling, and revised ...
Removed some debug cruft from `objects_scan()`.
Merge pull request #5 from asr1/master

Merging changes to rover's comm protocol
Revised `comm.py` to reflect current understanding of communications protocol.
Revised comment in `comm.py`.
Merge pull request #1 from dwtj/master

Revised comment in `comm.py`.
Implemented `frame_data()`.
Added official message protocol
Implemented `receive_message()`.
Added function to read data frames
Fixed `heartbeat()` and `ping()`.
Replaced tabs with four spaces each
Merge branch 'master' of git://github.com/aore/mars-rover into aore-master

Conflicts:
	control/comm.py
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
Merge pull request #8 from asr1/master

Pull of Alex and Flavia's initial work on reading from bluetooth.
Fixed struct packing in `servo.pulse()`.