Easier feature detection

Hi,

Currently, using this normalization, ``navigator.getUserMedia`` always exists and is always a function. This makes feature detection a little bit intricate.

This commit makes feature detection easier by verifying whether or not ``navigator.getUserMedia`` is supported and by setting ``navigator.getUserMedia`` to ``false`` if it is not supported (instead of always being set to the normalization function).

We can now do something like this:

```
if ( navigator.getUserMedia ) {
  // Use normalized navigator.getUserMedia.
} else {
  // Whoops, getUserMedia is not supported.
}
```

Thanks, anyway !