Add @dynamic property for UIActivityIndicatorView that solves memory lea...

Running Instruments on an app where I'm using this category on imageViews inside cells of a collection view I noticed multiple leaks that should be solved with the proposed pull request. Basically, I added a dynamic property to keep track of the UIActivityIndicatorView that wasn't correctly released from memory (probably because of some 'timing' issue - the view wasn't deallocated in time?). With this patch Instruments doesn't show leaks anymore.