Merge: nitweb: introduce angular skeleton

This PR introduces the angular-js skeleton for nitweb. It contains only one line of nit, the other modification only html/css/js code.

For now, most of the views/controllers are empty and the interface only displays the mdoc of each mentity hilighted with nitlight/doc_down.

Some demo pages:
* http://nitweb.moz-code.org/package/popcorn
* http://nitweb.moz-code.org/module/core::ropes
* http://nitweb.moz-code.org/class/core::Array

Pull-Request: #2142
Reviewed-by: Jean Privat <jean@pryen.org>