sensors: Support for up to three sensors of the IMU types
LSM303D: Support for tertiary sensors
HMC5883 driver: stack size adjustments of start handler and start return review / comments
L3GD20 driver: stack size adjustments of start handler and start return review / comments
LSM303D: start handler fix and start routine review
airspeed drivers: Start handlers stack fixes and start handler review
MPU6K: Start handler startup and stack review and adjustments
Merge pull request #1187 from PX4/mavlink_strict

Mavlink strict
Merge branch 'master' of github.com:PX4/Firmware into sensor_startup_cleanup
Making lowpass filter init bullet proof
device driver: Fix compile warnings
LSM303D: Ensure init and constructors
L3GD20: Ensure init and constructors
HMC driver: Full initialization
airspeed driver: better init
I2C driver: forbid copy constructor
MEAS airspeed: Stricter initialization and constructors
MPU6K: Cleaner init
FMU driver: stricter init
IO driver: stricter init