Added functionality that will return the next best quality in case the desired quality cannot be found instead of just returning an error.
Block and delegate callbacks will return the video quality allowing the developer to choose whether to use the URL returned in case the quality didn’t match the desired quality. For example if you ONLY want HD streams, but they aren’t available the library will now return a URL after all, but tell you the quality so you can make an educated decision :)
Updating README
Merge pull request #6 from runmad/master

Return lower quality instead of error if desired quality doesn't exist