Experimental! Added first video_input prototype and introduced the Media class to abstract all further access to Media devices
* new VideoInput class allows adding canvas elements that are then treated like a video device similar to the DeviceAPI
* a new canvas can be added as video device via Media.SharedInstance.VideoInput.AddCanvasDevice("canvas", canvas);
* the new Media class delegates getUserMedia and GetVideoDevices either to DeviceApi or VideoInput depending on the device names used
* the Unity plugin uses the Media class now instead of the DeviceApi
* On startup the unity plugin will get the first canvas element in the document and add it to VideoInput for testing via Unity (there is no C# API yet)
* added tests for the new features + general cleanup to allow the tests to run in firefox with fake media turned on