Add support for creating docsets

Instructions from http://kapeli.com/docsets#dashDocset.
The javascript is currently pretty janky inside dash.
We'll probably want to keep everything expanded when inside a docset, as well as hiding some of the chrome (especially the sidebar). @orta, any advice on doing that?

@kapeli, is there any chance you could look this over and let me know if I've done things correctly?
Apple's references docs are also all expanded when viewed in Xcode and Dash, so this is fine.