ArduCopter: Battery failsafe triggers RTL only if we are more than 15m from home, otherwise it performs a LAND

Bug fix from Leonard.

All changes by Sandro Benigno

Updated AP_Camera class.

Thanks to Jonathan for this.

ArduCopter: bug fix to GUIDED mode.  Now checks that desired altitude was reached before deciding the waypoint is complete.

ArduCopter: add AUTO_VELZ_MIN, AUTO_VELZ_MAX and PILOT_VELZ_MAX to allow better control of climb/descent rate in auto and manual throttle modes

GPS_PROTOCOL_MTK16
to
GPS_PROTOCOL_MTK19

ArduCopter: move gcs_check function (which sends to ground station) to run when 50hz loop is not running

Also removed redundant heartbeat message

Protected behind a #define

Filter: #include AP_Buffer.h no longer needed now that completementary filter has been moved to InertialNav library

ArduCopter: stop unnecessary updates of target altitude and knock-on effect of too many EV message in dataflash log