Document Formula attributes

The beginnings of some decent API documentation. Mostly here for visibility; will just merge this if there are no comments in the next few days. Trying to give people a sense of the style I'm hoping to go for in the rest of the Formula file.

Once merged it'll show up http://www.rubydoc.info/github/Homebrew/homebrew/master/frames

Also:
- tweak `.yardopts` to better fix our internal style
- add `install` method (mostly for links in API documentation and actual documentation later)
- protect some methods that aren't used publicly to avoid documenting them (and weird access of them)
`initialize` is special-cased by the runtime to always be private.

The rest should probably be private rather than protected (as there isn't any reason another instance of formula should ever call them).