Don't check for test releases by default

[drvdijk]

When downloading the great iTerm2 from iterm2.com, it directly asks me to update to a new version.
Turns out the app uses the test release line by default.

[gnachman]

Unfortunately, changing this flag would move almost all users out of the beta track and into the stable track. At least some of them want to be beta users, and I can't afford to lose everyone. Additionally, the master branch is beta-only at the moment, although a change like this would be appropriate in the v2 branch.

It looks like new users get prompted the first time Sparkle tries to auto-update. -[SUUpdatePermissionPrompt promptWithHost:systemProfile:delegate:] presents this dialog. I think it should include a checkbox asking if the user is interested in test releases. Whether to show the checkbox and the action taken if it's is checked (or not) should be done through the SUUpdaterDelegate interface. iTermController could implement that protocol and make itself SUUpdater's delegate.

My branch of sparkle is at github.com/gnachman/Sparkle if you want to take this on. Otherwise I'll get around to it when I have some time.