Make buttons themable

It would be great if buttons were theme-able and over-ridable by themes. Drupal.theme() javascript function provides support for this.

The attached patch moves the two hard-coded instances of <a href="javascript:void()"></a> into Drupal.theme.prototype.jCarouselButton(). This allows themes or other modules to override the markup which is used for the previous and next buttons, adding an <img>, custom text or other content which can not be added solely via CSS.

This feature will be especially useful for #825692: Pager or navigation for the carousel.

Although this feature/patch does not depend on #825692.