hotfix: use SNTP time for position sensor
Created generic classes for periodically polling sensors
Created generic classes for periodically polling sensors
Fixed reliability issues for magnetic field sensor
Minor code flow fix.
Changed MagneticFieldSensor to singleton.
Merge pull request #24 from senseobservationsystems/magnetic-reliablility

Magnetic reliablility
Merge branch 'unstable' into release-2.6.0

Conflicts:
	src/nl/sense_os/service/SenseService.java
	src/nl/sense_os/service/ambience/MagneticFieldSensor.java