Allow relative URLs in notebook extensions

When loading notebook extensions, additional resources (like CSS) need to be specified as absolute URL. This forces people to put an extension in a prefined directory structure or they have to modify the javascript code to change the path.

Now require.js allows URLs relative to the module:
```Javascript
define(["require"], function(require) {
    var cssUrl = require.toUrl("./style.css");
});
```
(see: http://requirejs.org/docs/api.html)

Unfortunately this does not work, like a simple example demonstrates:
```Javascript
/* testmodule.js */
"use strict";

define([
    'base/js/namespace',
    'jquery',
    'require',    
], function(IPython, $, require) {
    console.log('Testext')
    require(['./submodule'])
})
```
If loaded as notebook extension, instead of additonally loading `/nbextensions/submodule.js`, the invalid path `/notebooks/nbextensions/submodule.js` is used.

This PR allows to use relative URLs by providing the correct path to '/nbextensions'.

Wouldn't just making the prefix `/nbextensions/` do the trick?