Various minor fixes

- Added casts and method declarations to keep older compiler from showing warnings
- Release IMBAppleMediaParser instance variables in the dealloc method of the class that declares them
- Check CGImageSourceStatus to prevent crashes on incomplete files
- Use setValue:forKey: rather than setObject:forKey: when we cannot be sure the value is not nil
- Don't use NSInteger for results from calls that are declared to return 32 bit ints