Specify an empty dependencies set in package.json

NPM goes really crazy without this. You'll get a lot of
called_on_non_object errors when trying to perform (even
global) install operations.
Extensible block definitions in layout.html

The following new blocks have been defined around/within elements of the
layout so that people using the theme can extend it in certain places
without copy-pasta'ing the whole layout to make additions.

head
document_body
    sidenav
        search
        index_link
    content_wrap
        topnav
        content
            body_wrapper