Implemented a HMDCamera class, got rid of the CompositeViewer

Hello, 

I have managed to get everything working using a normal Viewer, so the CompositeViewer isn't needed. I have implemented an HMDCamera class, so adding Rift support in an existing application is very simple now - just instantiate it and provide it with a View and OculusDevice to work on. 

The reason why I have put OculusDevice as a parameter is that I am thinking about different OculusDevice subclasses - e.g. one providing dummy data (even when no Rift is available) for testing and another using e.g. VRPN for tracking the HMD. In that way the rest can remain the same. 

Jan