Jwg real timer

This PR gets rid of a hacky, self-made timer implementation. Instead, it uses an `NSTimer` to have more accurate timing.

This changes quite a bit of the internals of `JWGCircleCounter.m`, so please be careful if you've made any internal changes. (the header file, `JWGCircleCounter.m`, should be the same as before)

If anyone is interested, feel free to and add comments.

Thanks to [this answer](http://stackoverflow.com/a/6765300) for some guidance on how to do this.