Bulk render select multi choices to speed up rendering

- Updated MultiSelect.addSelectedChoice to return a string rather than a
  modifying the dom directly.

- Changed updateSelection to add the built up html string into the dom
  in one shot

- Added function bindCloseEvents to bind all of the close events to the
  close class.

- Store data about the selection in data-select2-data attribute on the
  element rather than in jQuery.cache via .data(). This was necessary to
  allow the unselect method to remove the unselected value properly