[Sparkle] Sparkle 2 launchCount

[Josef W. Wankerl]

Hi,

I think I'd prefer the SULaunchCountKey inside SURegistrar.m to be a
countdown instead of a count up.  It'd start at the
launchesRequiredForPrompt value if it doesn't exist, and then count
down to zero.  When it transitions from one to zero, then it'd be time
to attemptInstallation.  If it already exists and is zero, then
nothing happens.

That way the app plist file isn't always changed upon every launch of the app.