plugins: sys_status: Add MEMINFO handler

MEMINFO from ardupilotmega.xml message definition.
Optional.
plugins: add some header doxygen tags

Add license to Dummy.cpp (plugin template).
plugins: sys_status: Add SYSTEMTEXT handler

Two modes:
  - standard MAV_SEVERITY values
  - APM:Plane (default)

TODO: add mavlink dialect selection option
Fix build and update MAVLink library
plugins: Add GPS plugin

SYSTEM_TIME to TimeReference support.
TODO GPS fix.
node: Terminate node on serial port errors
mavconn: small debug changes

Limit no GCS message to 10 sec.
plugins: gps: Add GPS_RAW_INT handler

GPS_STATUS not supported by APM:Plane.
ROS dosen't have standard message for satellites information.