No need to have a public `releaseVideo` function.

The `stop` function should be suitable for all use-cases we can
currently conceive...
Don't die if a browser doesn't support MediaStreamTrack.getSources

It looks like currently Chrome is the only browser to support this
API. Unfortunately client code will need to check the length of the
returned array and behave accordingly...
MediaStreamTrack.getVideoSources is not crossbrowser

We now deal with this and the API is a bit cleaner (not expecting a
`sourceId` when you `prepareVideo` since you can't provide this on
most clients)
The default example doesn't let you choose cameras and introducing
a new example which lets you select camera...
Removed unnecessary debug code