AP_Mission: replace block read/write for eeprom

Also reserve command #0 for home position
Copter: add AP_Mission object parameter

Remove global command_total and command_index parameters
Copter: instantiate AP_Mission object
Copter: remove low-level handling of mission cmds

Now handled by AP_Mission
Copter: add mavlink packet to cmd conversion to GCS_Mavlink.pde
Copter: ch7 save wp feature to use ap_mission
Copter: log using mission commands
Copter: process cmds from mission lib
Copter: remove unused definitions
AP_Mission: special handling for cmd
Copter: pass ahrs into mission constructor