Add delegate callback when user makes choice to install / dismiss / skip an update

[zorgiepoo]

We already have one for skipping:

- (void)updater:(SPUUpdater *)updater userDidSkipThisVersion:(SUAppcastItem *)item;

Might as well add one for dismissing. Also we should pass the user update state, potentially deprecate this skip one (for a newer one with user update state), and use "update" instead of "thisVersion" ..

edit: actually, I need to give this more thought.. since this isn't when the update cycle ends, but maybe that is when the delegate actually cares.