Get information about available updates

[lanwen]

There is a method CheckForUpdateInformation in the binding, however, it's effectively a no-op, since no callbacks are propagated into the go part:

 `-[SPUUpdaterDelegate updater:didFindValidUpdate:]`
 `-[SPUUpdaterDelegate updaterDidNotFindUpdate:]`

these methods have to be implemented on a delegate which could call a CGO function to propagate that to the main Go application

It would be quite useful to have these bindings, as right now any UI representation of the available update requires a separate implementation, which would duplicate the sparkle functionality.

The initial implementation for the delegate could be found in #24 with an example of calling a cgo callback.