Merge changes from main repository into FlyBlind

Merged changes from the main repository:

- Added a separate volume control for speech.
- Alarms are relative to ground elevation, rather than sea level. Ground elevation is specified separately in the configuration file.
- Instead of processing UBX messages as they arrive, we now process everything once a complete set of UBX messages has been received.
- Green LED turns off immediately when the log file begins. This allows us to accurately synchronize video with the FlySight log.

You might consider merging the ```DZ_Elev``` and ```Elevation``` configuration parameters, but I thought I would leave this for now since I thought it would be confusing to have the ground elevation specified in the FlyBlind parameters separately from the alarm parameters.