Kinect sensor coordinate to color image coordinate conversion

`ofxKinect::sensorToColorCoordinate(const ofPoint & p, float distance)` converts a sensor (depth w/o registration or IR image) coordinate to the corresponding color image coordinate. The other way round is not possible since this conversion requires a depth value.

I was only able to test with osx10.9.4+Kinect1473, and maybe there are some issues with other Kinect models so it should be tested before merging.