Added support for setpid

Example: setpid --pitch -p 5.0

Support is not fully fleshed out because the quad
doesn't support everything. Some backend changes
will be needed to support the rest of the PID constants that
are in the design document.

Right now, the P and the D of pitch, roll, and yaw are
supported. The Height/Lat/Long PID controllers are
not supported by the quad<->backend protocol,
nor are the I constants of anything. The quad<->backend
protocol is aware of a Throttle controller, which is
nowhere in the documentation.