camera.FlyControl for highly configurable FPS or flight-sim style camera control (#266)

* fleshed out basic functions of FlyControl and keyboard controls.

* minimal functionality to work in test program (g3nview demo)

-some 'default' speeds, keys, constraints. to be options later.
-apparently RotateOnAxis() doesn't work right on the camera?
 I had to use LookAt() to change the camera's angular direction.

* mouse control of FlyControl. still needs polish.

* support mouse config for when mouse is and is not captured.

* configuration options, switching between using camera up or world up.

* documentation

* added camera.FlyControl to provide a highly configurable FPS or flight/space sim style camera control.

* remove unused function stub

* remove commented subscribe lines

Author: quillaja