add low pass filter to meas airspeed driver including logging of filtered value
Merge remote-tracking branch 'upstream/master' into diff_press_filter

Conflicts:
	src/modules/sdlog2/sdlog2.c
fix logging of filtered diff pressure signal
Merge remote-tracking branch 'thomas/diff_press_filter' into test_20140315
Merge branch 'test_20140315' into test_20140315_paul

Conflicts:
	src/modules/sdlog2/sdlog2.c
	src/modules/sensors/sensors.cpp
	src/modules/uORB/topics/sensor_combined.h