Support __onGCastApiAvailable

[Lindsay-Needs-Sleep]

It would be nice if we could implement chrome Api's method of signalling that the api is available.

In Chrome desktop you do:

window['__onGCastApiAvailable'] = function(isAvailable, err) {
  if (isAvailable) {
    // start using the api
  }
};

But in cordova-plugin-chromecast you do:

document.addEventListener("deviceready", function () {
    // start using the api
});

Would be nice to conform a bit more because:

• less difference between our usage and chrome's = less documentation we have to write and maintain. :D
• Allows identical code to be ported between web and cordova
  • (This happens to be my use-case. We use a remote web server which works from desktop and cordova.)