Mission: add p1 to command structure

mavlink_to_mission_cmd method extended to support plane including adding
parameter to allow the alt to be expressed in cm or meters
Common: remove id and p1 from Location structure

id and p1 are now part of AP_Mission's Mission_Command structure
Copter: integrate move of p1 from Location to mission cmd
Mission: update example sketch for p1's move to cmd structure
AHRS: interate move of p1 from Location to mission cmd
Copter: replace memset in GCS_Mavlink
Plane: add AP_Mission object to parameter list
Plane: dataflash logging to use Mission command
Plane: mission integration into GCS_Mavlink.pde
Plane: integrate AP_Mission library