Added an XPC service to deal with sandboxing.

This fixes #163. It requires that you copy the XPC service into your main application bundle in
Contents/XPCServices and runs the remove from quarantine step of copying the finish_installation
app as well as launching finish_installation app outside of the sandbox. Sparkle will function
exactly as it does now for applications that do not copy the XPC service into their application
bundle.
Merge branch 'master' of github.com:wbyoung/Sparkle

Conflicts:
	Sparkle.xcodeproj/project.pbxproj
	finish_installation.m
Merge branch 'master' of github.com:andymatuschak/Sparkle