add man page brew-cask.1

- the change to the Formula needs checking/testing
- the source at `doc/src/brew-cask.1.md` must be kept in sync with `USAGE.md` and other docs in the future
- the compiled form is also kept in the repo and must be regenerated after changes
- `ronn` is required to compile the manpage
- a shell script is provided to generate the compiled page, but that should probably be folded into the Rakefile