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
Plane: bug fix for relative alt
Mission: remove alt_in_cm from mavlink to cmd conversion
Plane: remove alt_in_cm from mavlink to cmd conversion
Copter: remove alt_in_cm from mavlink to cmd conversion
Mission: set_current_cmd to use uint16
AP_Mission: fixed reset using set mission item to 0

this causes a mission reset
Plane: fixed altitude for takeoff and takeoff heading

fixes for AP_Mission integration

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
Mission: allow set_current_cmd when mission is not running

Can now be run when mission is complete or stopped.  When mission is
resumed it will begin at the set command.  This command can be either a
nav or do command
Mission: example sketch test for set_current_cmd
AP_Common: pack Location structure

This resolves issues storing and retrieving Location to eeprom as part
of AP_Mission