Add Apple command-line tools

This should add a sensible "SDK" and precompiled command-line tools from ADC, possibly suitable for bootstrapping nix on darwin someday. You'll need to configure an `adc_user` and `adc_pass` to download it. Even if we don't use the binary tools for bootstrapping, having the SDK in the store will be quite valuable when automating calls to `xcodebuild`, which will be necessary for building all sorts of nice packages from opensource.apple.com.

@shlevy, wanna check it out?
Good call :)