Introduce "$.contains"

From [the jQuery API docs on
$.contain()](http://api.jquery.com/jQuery.contains/)

> Description: Check to see if a DOM element is within another DOM element.
>
>     jQuery.contains( container, contained )
>
> - **container** The DOM element that may contain the other element.
> - **contained** The DOM element that may be contained by the other element.
>
> **Note:** The first argument *must* be a DOM element, not a jQuery object or
> plain JavaScript object.

Add additional fixture to more thoroughly test the logic for nested structures.

Add to project documentation.
Merge pull request #140 from jugglinmike/contains

Introduce "$.contains"