test(ionCheckbox): add ngChange test
fix(activator): properly activate elements nested inside an item.

Addresses #1373.

Before: if an `<a>`, `ng-click`, or `<button>` inside of a .item,
was clicked, the activator would walk up .item and activate it.

Now:  If an `<a>`, `ng-click`, or `<button>` inside of a
.item is clicked, the activator will activate that element.

Additionally, CSS was added so `a.item-content.activated` would look
activated.