web: added icon for SiK firmware
web: fixed icon
Copter: reduce autotune min rate P to 0.01 (was 0.02)
GPSGlitch: reduce radius to 2m
Copter: AC3.1-rc8 release notes
Copter: reduce autotune min rate D to 0.002
AP_InertialSensor: added support for multiple accel/gyro devices

this makes it possible to ask for the gyro and accel vectors from
secondary INS devices.
AP_InertialSensor: implement up to two sensors on PX4
DataFlash: added Log_Write_IMU2() for logging 2nd INS sensors
PX4: start all sensor types on PX4

this allows for secondary sensors
Copter: log both primary and secondary INS samples
AP_Math: added is_zero() method to Vector3
AP_InertialSensor: generalise the accel/gyro calibration for N sensors
DataFlash: fixes for INS API change
GCS_MAVLink: update from upstream XML
GCS_MAVLink: re-generate MAVLink headers
AP_Compass: switched to a vector based interface

this is more consistent with other APIs and makes multi-device support
easier
Rover: updates for compass API change
Copter: updates for compass API change
Plane: updates for compass API change