Update select2.js

Fixed issues with select2 and disabled options, most of it having to do with incorrectly determining the index of an option when disabled options were being displayed.
Update select2.css

Made disabled results show up disabled, and added a class "selected" for elements that have already been selected (by default, these elements are hidden).
Update select2.js
Update select2.css
Merge pull request #747 from Kollibri/master

Fix issue with disabled options (Issue #733)