use mono calibrators mkgray in cameracheck.py, some variable renaming, 

fixes ros-perception/image_pipeline#31

Somehow the conversion of mono16 to bgr8 did not work, the [two-stage approach of cameracalibrator.py](https://github.com/ros-perception/image_pipeline/blob/groovy-devel/camera_calibration/src/camera_calibration/calibrator.py#L252-L271) does work though.