Convenience for making `Result` from `NSErrorPointer` API

Cocoa API commonly uses `BOOL` or object type returns coupled with an `NSError **` parameter where we would like to use `Result` instead. This provides a pair of conveniences to produce `Result` in those circumstances.

This also brings in a deployment target for iOS.
Should this be `Result<()>` rather than `Result<Bool>`? See @bradlarson's code at https://github.com/LlamaKit/LlamaKit/issues/10#issuecomment-64083613 for an example of that.

I agree that there are advantages of this over `NSError?`. Unit is a legitimate thing to handle.