Be able to discover multiple APIs

We released this library with dynamic generation on runtime by making a request to Discovery API and but it's not feasible for short-lived scripts. We should cache discovery response or generate a object and service layer during build process.

I'm suggesting to replace the existing loader with the following interface that allows to discover more than one API and skip cache.

	googleapis
	  .discover('plus', 'v2', opt_opts)
	  .discover('urlshortener', 'v1', opt_opts)
	  .withApiKey('...') // optional
	  .execute(function(err, client) {
	
	  });