Missing documentation for what concerns camera sensors frame

I was unable to find information about the default direction of the optical axis of a generic camera sensor.

Without explicit information, I was assuming:
- z-axis goes along the optical axis (pointing outward)
- x-axis points rightward (wrt to the sensor's image plane)
- y-axis points downward.

(See [here](https://www.ros.org/reps/rep-0103.html#suffix-frames) for reference)

But looking at a simple example like [this one](http://gazebosim.org/tutorials?tut=camera_distortion&cat=sensors), in absence of any `` specification, the optical axis seems to be along x, since for Gazebo Z is up and X is in front then the camera plane is defined by YZ instead of XY as I was assuming.

Can you confirm that?

For the users, it would be ideal if the documentation could specify the documentation should specify on which axis the optical axis is oriented, and which axes define the image plane.

Related downstream issue: https://github.com/robotology/icub-models/issues/69

cc @traversaro