Sensors: add support for US Digital MA3 encoder, which is used for angle of attack sensing.

- PWM output from MA3 can be read on the PX4 and converted to an angle in radians.
- MAVLink message "Aerodynamic_variables" containing angle of attack, sideslip angle, and airspeed.

Major limitations:
- Requires PX4 firmware updated. This patch relies on some new modules that have been
	implemented on the PX4 codebase. It won't compile without them.
- Untested so far, will do that in a few days.

Lesser limitations:
- On the APM side, only the PX4/PixHawk boards are supported. Support for the others will
	depend on what pins/timers are available.

To-do list/other notes:
- common.xml changed slightly because it was breaking mavgen.py.
- code layout to be checked.
- testing to be done.
Is it just me, or is Git a huge pain in the neck on a daily basis?