add man page brew-cask.1

[rolandwalker]

• 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