Merge pull request #1 from dwtj/master

Merge David's Ping
Refactored errything. Moved everyting outside of Control to respective subsystem files, deleted COMM.H. PING works.
Outlined ISRs, no longer compiles probably. Did some refactoring, probably. Echo probably works.
Fixed sonar distance conversion and reverted from linear width to angular width for object detection (i.e. Lab 9).
Merge https://github.com/asr1/mars-rover into asr1-master
Re-added `aux.py`. It keeps getting deleted for some reason!
Reimplemented `aux.py` *much* more cleanly as `sentinel.py`.
Revised `comm.tx_mesg()` and `comm.rx_mesg()` to use `Sentinel`. Also:
- Moved communication protocol codes into `codes.py`
- Added helper functions to `Sentinel` for reading and writing integers.
Finished initial (i.e. untested) changes to `comm` and `sentinel`.
Merge pull request #7 from dwtj/master

First draft implementation of `sentinel`.