Dictionary Ordering

@vokalinteractive/ios-developers Code review please?

Also, will need to be tagged `0.1.1` when it's merged.
I was tempted to add a category on `NSData`, but I thought the (slight) benefit versus the (slight) risk of collision and complication from the category in the library wasn't worth it.

Ultimately, the problem is that I'm not sorting based on the objects that are actually in the array—I'm sorting the actual dictionary keys (`NSString` or `NSData`) in the array based on the somewhat-normalized `NSData` objects those keys map to in the `keyDataByKey` dictionary, so whether the comparator is a function or selector, it's still not set up to directly handle the objects in the dictionary.

It does feel messy, though, so I'm open to suggestions.