feature(elgg_require_js): add fallback for require.js configured with CDN.

There is a fallback with require.js. It could be useful to use it when we want to register a CDN and improve performance. `elgg_register_js` actually not provide it.  
They talk about how to do it [in the wiki](https://github.com/jrburke/requirejs/wiki/Upgrading-to-RequireJS-2.0#wiki-pathsfallbacks).

With this feature, we can add a second source like that:
```
elgg_register_js('chart', array(
	'src' => array(
		'//cdnjs.cloudflare.com/ajax/libs/Chart.js/0.2.0/Chart.min.js', // cloudfare CDN
		'/mod/myPlugin/vendors/Chart.min.js' // local fallback
	),
	'deps' => array('jquery')
));
```
**Old method still works!**
```
elgg_register_js('chart', array(
	'src' => '/mod/myPlugin/vendors/Chart.min.js',
	'deps' => array('jquery')
));
```