Added `lang` attribute support

Select2 will now detect the language based on the `lang` attribute
of the `<select>` element or any parent elements.
Allow for custom sorting functions

This adds the `sorter` function, which works in the same way as the
old `sortResults` function.
Add `minimumResultsForSearch`
Backspace unselects item in multiple selects

The backspace key now behaves very similar to how it does in other
multiple select boxes, such as the one that Stack Exchange uses.
When the user presses the backspace key, the last selected option
is unselected and the search box is filled with the text of the
option.
4.0 announcement