Add bb-edit cask for installing BBEdit

As an aside, and relating to #365, the naming of both the file and the class feels weird on this. Just as my 2¢, all technical concerns aside, `bbedit.rb` with a class name of `BBEdit` would have felt most natural here, to me. But that [did not work](https://gist.github.com/benspaulding/f457c60a9aa2c852b004). But this works for now until #365 gets sorted out.

And thanks for all your work on cask — it is a great utility.