Remove static code from YawRateController_gains.cpp.
Remove unused constants.
Move constants to a cpp file.
Implement Encoder class.
Implement Motor class.
Add minimal firmware test framework.

This commit also adds constants.cpp to the Makefile, which should have been
done in a previous commit.
Fix Motor constructor parameter names.
Add Newton * meter to ounce force * inch conversion.
Change counts per revolution constructor parameter to uint16_t.
Simplify Motor constructor and destructor.
Change test_motor() so both motors are tested.
Fix constant declaration.
Eliminate encoder_priv.h.
Eliminate motor_priv.h.
Eliminate status shell command, rename disablemotors shell command.
Update shell commands and tests.
Initial fork motor controller implementation.
Initial rear wheel motor controller inmplementation.
Initial motor controller abstract base class implementation.
WIP: Replacing old yaw rate and rear motor controllers with new ones.