Added documentation footer

The documentation will be licensed under CC BY 3.0, instead of
under the MIT license like the rest of the code.
Added support for a click mask

In past versions of Select2, a mask was used to detect clicks
outside of the dropdown. While this works with high accuracy, and
avoid many of the click issues we had with modals, it is no longer
the default close handler for Select2. It blocks any features on
the container from working without a second click, and introduces
odd edge cases that we cannot easily handle.