Add new documentation lookup command (Mavericks)

The implementation has changed so that it is now possible to use “back” to go back to previous page, it also auto-links things that look like method symbols.

Presently though it does not auto-link methods listed in the includes, class, and instance method sections. Also, it shows the markdown output from ri pretty much as-is.

It uses the version of ri included with OS X 10.9 (Mavericks) so the command is only made available to people on 10.9.