Add Recenter functionality

[cai-mobfish]

Detail

This is a standard Recenter function, it can be used to make camera face "Front" in scene.

[rohantipare]

@cai-mobfish
I have implemented the Recenter functionality and pushed on branch 'recenterCamera'
Function: RecenterCamera() in CardboardHeadTracker.cs
Scene: 'demoScene - test recenter'
The demo script recenters the camera every 5 seconds.

[cai-mobfish]

@rohantipare
Thanks for the implementation, this recenter works, but we also need another kind of recenter, that the camera only "recenter horizontally", can you do an extra method for this?

Also, I did a little change in this branch, the DemoScene also contains a button to recenter now. You can pull this branch before making another changes.

[rohantipare]

Sure, I will implement the recenter horizontal only functionality

[rohantipare]

@cai-mobfish I have implemented the Horizontal only recenter. Changes are pushed to branch 'recenterCamera'

[cai-mobfish]

Tested, works.
Did an extra commit to remove unused files.

Merged into public-dev.