add support for annotated tags

This is support for tags as far as I need it.  I'm new to go and libgit2 but I tried to orient myself on your code.  Please have a look.
I've come up with http://play.golang.org/p/_oabRa8CxE which is a bit of a monstrosity but it does mean that we can reach the `gitObject` without having to check the type explicitly.