The callbacksForURL: method in SDWebImageDownloader should return a copy of the mutable array!

Since the callback array is used for fast enumeration and could be mutated via removeCallbacksForURL, a copy of the mutable array should be returned instead of the mutable array reference. It is also a recommendation since the method signature indicates the return type as NSArray.

This should fix the crash in multithread environment such as:
"Collection <__NSArrayM: 0x226d9310> was mutated while being enumerated."

This should fix the #470 issue.