Allows the img tag selected by matching the media to have a specific class

Adds a data-img-class attribute to the span[data-src] element which optionally allows the developer to have a class attribute set on the img tag that it added to the DOM.

This requirement came from working on a project that uses [SMACSS](http://smacss.com/) which suggests (but does not enforce) that modules do not use HTML elements in the styles.

This therefore allows the resultant img tag to be targeted using CSS classes rather than the img element.