A simple ADC downloader for upcoming darwin goodness

ADC = Apple Developer Connection, where they host all their SDKs and other passworded downloads.

It assumes `adc_user` and `adc_pass` in `config`. Not sure of the best way to do this sort of thing, but this seems like a decent starting point. We can now use this to stuff xcode and friends into the nix store and package them nicely.

**Reviewers: please make sure that I'm doing the right things to prevent hydra from caching things downloaded from ADC. I'm setting unfree license on the output, but I'm not sure that's sufficient.**