Fix issue with disabled options (Issue #733)

Added a new class called ".select2-results .select2-selected" to replace the old ".select2-results .select2-disabled".

Uncommented the css that was for displaying the "select2-disabled" elements.

All places where select2 was trying to get the index of the current select2-result element, had it look within all visible results, not just results there were not disabled (because sometimes we are displaying disabled results).

When an element gets selected, added the class "select2-selected" to it,instead of "select2-disabled".

This is my first time contributing to a project, so if I screwed up, please tell me what I did wrong so I can fix it.