Update camera poller to close the camera when polling is complete.

[JeetShetty]

#117

[coveralls]

Coverage increased (+0.1%) to 80.662% when pulling a5f5608 on close_camera into 1d3d410 on master.

[mtlynch]

LGTM just one note

[mtlynch]

Suggest:

        self._sensor.close()
        super(_CameraPollWorker, self).stop()

Because then if we ever add anything to the base class' close, we don't have to remember to add it here independently. Also I'm suggesting calling the parent's close second because it's the opposite of a constructor. The constructor we want to call the parent first because the child might have dependencies on the parent's object initialization. So when we close, we want to close the child's things first, then close the parent's in case the child has any dependencies on the parent's objects staying alive.

[JeetShetty]

Oh yeah! Good points.

[coveralls]

Coverage decreased (-0.005%) to 80.556% when pulling cbff78a on close_camera into 1d3d410 on master.