Container for loader and trigger components (Fixes #97)

Current version of library uses **div** to wrap the *loader* image or *trigger* button. These components are added after the last item inside the container.
In order to be able to support a larger number of components as container and item, the html element used to wrap the *loader* and *trigger* should be configurable.

Indeed, if you use an `<ul>` as the container and an `<li>` as the item, the loader image and the trigger button will appear as `<div>` resulting in a non standard DOM construction (`<ul><li /><li />...<li /><div class="ias-trigger">...</div></ul>`) and difficulties (depending of the browser) to manage presentation through css.

This pull-request adds a new configuration parameter: **control_container** that will hold the new wrapping html tag name for the *loader* and *trigger*.

  * **"div"** will be the default value (as current code).
  * An **"auto"** magic value will be also supported to let the library to use the same html tag name as the one holding the item selector.