AP_GPS: added support for GPS time in week/millisec

also adds time_epoch_usec() for MAVLink SYSTEM_TIME
HAL_SITL: properly emulate timestamps on ublox and MTK
AP_InertialNav: updates for new GPS API
DataFlash: updates for new GPS API
Plane: added MAVLink SYSTEM_TIME message
Copter: added MAVLink SYSTEM_TIME message
Rover: added MAVLink SYSTEM_TIME message