Improve handling of static resources in HTMLFileSession

This changes `js` and `css` to `resources` in `HTMLFileSession.save()` and related functions. Added 'relative-dev' and 'absolute-dev' ('inline-dev' doesn't make sense) options which allow to use `requirejs` instead of `bokeh*.{js,css}` files, so that you can develop with `grunt watch` for fast turnaround.