Plane: check for parameter send at default of 10Hz

50Hz can cause overruns, especially on PX4 which can drain the tx
buffer very rapidly
Plane: change default modes to FBWA not STABILIZE
autotest: added command line args to test script
AP_L1_Control: use a reference to AHRS

makes code a bit simpler
Plane: use reference to AHRS
docs: added doxygen support and doxygen build scripts
SITL: added simulated NMEA GPS

useful for validating the NMEA GPS driver
docs: fixed project names in doxygen output
SITL: update for new SIMSTATE message
AP_GPS: improved the precision of the NMEA driver

this brings the NMEA driver to the same lat/lon precision as the uBlox
driver (approx 1cm)
GCS_MAVLink: updated to latest upstream mavlink defintions
APM_Control: use a ahrs reference, not pointer

saves pointer check
AP_L1_Control: set min groundspeed to 0.1

this works better for slow rovers
Plane: update for new APM_Control API
AP_TECS: use a ahrs reference

saves a pointer access
Plane: update for new TECS API
SITL: support multiple instances of SITL running at once
docs: allow docs build to output to another directory

use DOCS_OUTPUT_BASE environment variable
autotest: build docs on each autotest run
autotest: added docs links