Target latest Android version
Updated Android Lint config
Added singleton pattern to magnetic field sensor
Only unregister the magnetic field sensor after a valid data point was
received.
hotfix: use SNTP time for position sensor
Created generic classes for periodically polling sensors
Merge branch 'unstable' into release-2.6.0

Conflicts:
	src/nl/sense_os/service/SenseService.java
	src/nl/sense_os/service/ambience/MagneticFieldSensor.java
Fixed error after merging MagneticFieldSensor
Only unregister magnetic field sensor when a data point was
actually received
Updated pressure sensor to use PeriodicPollingSensor interface;
changed pressure sensor data type to float.
Always unregister MagneticFieldSensor after max 500ms
Added SenseSensor interface, extended PeriodicPollAlarmReceiver to
handle starting/stopping the periodic polling