Install Homebrew on user's behalf

[ThatsJustCheesy]

Install Homebrew automatically at the user's request. Should be integrated into the dialog that appears when a Homebrew installation is not found (#5).

[ThatsJustCheesy]

I've opted to link to detailed instructions on the website, rather than attempt an automated install.

• the process is slow, so it's probably better to show exactly what's going on
  • I don't want to introduce a terminal to the setup window, as I'm trying to keep it fairly simple
  • so just sending users to Terminal.app makes the most sense to me
• users should be made aware of the storage space requirements/implications
• users should be made aware of the anonymized analytics and how to opt out—which Homebrew already does nicely