Workaround for Origin Trials

[spite]

webvr.info recently implemented Origin Trials, so the WebVR API is available for the page.

One side-effect of that is that, since the API is "loaded" when the page loads and the OriginTrials token is processed, it overwrites with native methods what the emulator has injected from the content-script, creating a mess of native and injected methods.

[spite]

I have a feeling this can be solved with #10

Rewriting as a wrapper that can work with the existing implementation plus one (or more) emulated devices, should work.

Also it means that it has to implement the wrapping lazily, to support this new possibility open by using OriginTrials.