Added HMD support for current WebVR implementations

input_hmd.js now prefers WebVR (Firefox Nightly or Chromium) over vr.js. This is just for getting the rotation of the HMD.

Known Issues:
- This still uses posteffect_oculus.js, not the native distortion that Chrome and FF implement.
- The HMD info values are all hardcoded for an Oculus DK2. This won't work for DK1 or other HMDs.