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?
In the Swift docset indexed by me, I use "Method" for subscripts and "Alias" for typealiases. I'm Swift illiterate though, so if you think I should switch to "Subscript" and "Typealias" let me know and I can add those types in the next update of Dash. // @jpsim