sensor fw backend wrongly calculates force of gravity

sensorfw accel backend wrongly calculates gravity as 9.812865328,
whereas the standard is 9.80665

As linked in the source:
http://en.wikipedia.org/wiki/Standard_gravity
(old code wrongly states wikipedia states it is 9.812865328)

This calculation is needed as accel data within sensorfw is handled as milli-G's (probably so it can use int and not deal with floats), and QSensors and the standard measurements are m/s^2