Basic support for {get,set}FrameRate() on iOS

Just a bit of code that lets you specify a slower frame rate on iOS, when 60fps is not required (or possible) and you want to reduce CPU usage.  This is my first Cinder pull request, so please let me know if I'm doing anything wrong with the process (and sorry in advance)!

It tries to work relative to any possible iOS device refresh rate, but as far as I can see, the only supported rate right now is 60fps.  I could have hardcoded this, but decided to be a bit more clever and request the current frame duration from the CADisplayLink object.

Thanks,
Glen.