Lazy loading graphs

This implements configurable (default off) dynamic or "lazy" loading of graph images with javascript to avoid excessive server load when showing pages so that images are only loaded when they are scrolled in view. When lazy loading is enabled, fallback for browsers without javascript with noscript tags is provided. An empty image is displayed as a placeholder for graphs to be loaded, with an approximate "average" size of the graphs so the page length doesn't jump very much when the real images are loaded.

Lazy loading is done with http://jquery.org and http://www.appelsiini.net/projects/lazyload plugin.

**TODO:**
* add lazy loading to the rest of the templates, for now only munin-nodeview.tmpl is done.
* ~~make the placeholder image prettier so it's more obvious the real images are still loading...~~

Comments welcome!