Allow for ESI-style caching

In order to use next-gen caching like ESI in Varnish, we need to allow for the cache plugin to alter the content, so that rather than the actual data itself, an ESI tag is returned. This would also be necessary to use NgineX's SSI+memcached system.

Currently panels_render_layout() does this for the whole page by saying $content = $cache->content; in the event that caching is triggered. We just need panels_get_pane_content() to do the same. It's a one-line fix. Patch attached!