added GPS check before mode change. New mode_requires_gps() function.
Rover: update docs on volt/current pin
Copter: update docs on volt/current pin
Plane: update docs on volt/current pin
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.
Plane: improve RTL altitude docs
AP_Param: add support for converting between old and new parameters

this allows for a conversion table between old parameters and new
parameters, making firmware upgrades with moved parameters much easier
for users
AP_Progmem: fixed type of strncmp_P()
APM_Control: fixed some documentation (old parameter names)
Plane: auto-convert the old RLL2SRV* parameters to the new CTL_RLL_* parameters
Copter: disable board voltage pre-arm check for PX4
Copter: pre-arm check for board voltage < 5.8V
Copter: pre-arm check for mag field length
Copter: fixed comments re mag field pre-arm check
Plane: fixed headers for mode logging
APM_Control: match parameter names with old PID controllers

this will make upgrading easier
Plane: removed the old roll/pitch/yaw PID controllers

use the new controllers, with auto parameter conversion
APM_Control: fixed parameter names
autotest: tweak plane autotest mission and parameters

a bit better with the new controllers
Copter Motors: minor formatting change