I used to have that built in; "Sparkle" is the name of the code framework that does that, but the latest version of Sparkle is buggy; I tried using the previous version that was used in Diamond 2.x but at the last minute before making the beta public my testers and I developed problems with it (update would signal user but not download and install). I jiggered with it but ultimately removed it since it literally doubled the size of the application to about 2.5 megabytes. I'm still working with the latest version though and as soon as I have it working I'll add it in.