9 DoF Sensor Accelerometer Readings Inconsistent
In 'FULL_RES' mode the number you get is amount of 4mg units. 

So 1 is 4mg, 2 is 8mg, -4 = -32mg.

Also note that your current code doesn't interpret the first bit (so the 13th from the right) as the sign bit, so you'll never get negative values. (Question related to this)

PS I think it is actually more like 3.9mg.