TECS : Initial implementation of TECS (Total Energy Control System) for integrated speed and height control

See pull request #260 for details
Plane : APM_control : Modified gain definitions for roll and pitch controllers so PID gains behave like previous servo PIDS

Gain definitions in roll and pitch controllers were updated previously so that the old PID tuning values could be transferred across.
Updated tuning guide for revised gain definition.
TECS-WIP : Changed default damping gain
Default damping gain was changed from 0.0 to 0.5. Results from flight testing indicated that baro delays were larger than modelled, requiring some damping gain. This default value will be refined with further flight testing on a range of models.
PX4 : Dirty hack to fix channel order difference when using SPektrum radio via S-BUS
Plane : Elevon Output Mixer : Removed /2 so that both channels can access full deflection individually
Hack : Temporary pitot source pin reassignment
TECS-WIP : Added compensation for additional induced drag in turns.

Also updated default parameter values based on SITL and flight test results.