Method updating/deprecation and documentation.

Really simple and useful display link! Thanks! My adjustments simply update the method declarations to follow the best practices suggested by Apple. The updates won't effect anyone currently using the object although it will bother them with a deprecation warning. I also updated the README file to reflect the new changes. The comments I added (using /**, */) display from Xcode's code completion and should further simplify the learning curve to using the Display Link.

Cheers!