Added Gitter badge
Notify: add pre_arm_gps_check flag

RGB LED will remain flashing blue when vehicle is disarmed and this
check has failed (i.e. false).
Copter: set pre_arm_gps_check flag
Plane: pre_arm_gps_check flag always true
Rover: pre_arm_gps_check flag always true
Tracker: pre_arm_gps_check flag always true
Notify: remove reliance on Led class
Notify: remove Led class

This class didn't add any functionality beyond what the NotifyDevice class
already provided so in keeping with our desire to keep things simple we
can remove this class
Copter: remove support for do-set-home mission command

This avoids the chance of an arithmetic exception in cases where the
position is moved too far
CHDK script changes

S100 settings
Copter: skip pre-arm checks when already armed
Copter: remove broken support for do-change-alt

This command was not implemented fully removing this code resolves a
compiler warning
AC_WPNav: minor const fix
Copter: disarm on landing regardless of pilot input
autotest: fixed bash syntax
AP_InertialSensor: prefer sensors that have zero error counts

if a sensor has failed in flight then try not to use it if another
error-free sensor is available
DataFlash: log gyro and accel error counts
AutoTest: increase throttle for fly_square test
AutoTest: minor fix to AVC copter params

remove unnecessary AHRS_EKF_USE because it's on by default now
AP_NavEKF: Increase rate at which GPS glitch offsets are reduced for planes