install Emacs.app

This installs emacs with `--cocoa` and bumps the version to `24.3-boxen2`. The manifest already tries to symlink the application so it seems like an oversight that it's not being built.

An alternate approach would be to parameterize the class, which would work but be kinda lame. Another would be `emacs::cocoa`. However, I prefer this approach as it's simple and we can bottle it up so people download a fully functional version in the first place.

Closes #3.