Deeply nested views with `el: false` are not inserted into the specified selectors.

Hi!

Due to [LayoutManager#partial](https://github.com/tbranyen/backbone.layoutmanager/blob/87b4c36fed2304624609124a0a9db8819ca5d60a/backbone.layoutmanager.js#L792) using only [jQuery.filter](http://api.jquery.com/filter/) to select the root element if the `el` was set to `false`, deeply nested views are not inserted into the specified selectors.

Example with the current version of `backbone.layoutmanager.js`:
http://jsfiddle.net/34NSH/2/

Example with my fix applied:
http://jsfiddle.net/qjnME/1/