Add link traversal support

[avaer]

Basically vrdisplayactivate events need to be emitted when you enter a site.

We actually already have a rough implementation of the emit, but we should clean it up to only emit once, at the right time, and preferably only when the HMD is actually nudged in the first place. I think OpenVR supports querying for this.

Additionally, it's worth thinking about how this should work with our reality tabs (tm :P) interface, where multiple sites are opened at once.

[shawwn]

This probably depends on #43.

[avaer]

No, this doesn't depend on #43. It's an entirely different WebVR-related issue.

Updated OP.