WebVR: Better availability check

[Mugen87]

This PR introduces a new .checkAvailability() function in in WebVR.js that uses a Promise in order to correctly check the availability of the WebVR API and HMDs.

.isAvailable() returns just a boolean to indicate if WebVR is possible or not. The Promise returned by .checkAvailability() will resolve if everything is fine. If something goes wrong, it rejects with a respective error message.

This approach solves an important issue: .isAvailable() does not indicate if WebVR is supported but no HMD is available. Besides the current workflow does not provide an appropriate error message in this case because the following code is never reached (the value of message is determined in a callback, while the actual function getMessage has already returned).

[Mugen87]

For testing: Please activate WebVR in your browser settings. The Origin Trial Token does not work with rawgit.

The "Enter VR" button works in the current implementation, but not the message toast.

With PR: http://rawgit.com/mugen87/three.js/webvr/examples/webvr_daydream.html
Current: https://threejs.org/examples/webvr_daydream.html

[mrdoob]

Thanks!