On close, modals should return focus to launching element

Created by: whicks1

When a user closes a modal, nothing gets focus. This breaks keyboard/tabbed usability (possibly accessibility?). Consider a long form that a user tabs through elements, and launches modals for things like help text. Ideally the last button or element used to launch the modal would receive focus back and the user could continue from where they left off.

Test: simply tab through http://getbootstrap.com/javascript/ and use only the keyboard to launch the demo modal, escape it, then start tabbing again to see the behavior in action.