keep original tabindex order

my ui has to work for keyboard-users (disabled people and power-users). what annoyed me about chosen was that i couldn't easily apply a style on :focus to the chzn-container.

that's why i manually add "focused"-classes to the .chzn-container when the search-input is focused:

```coffeescript
$(document).on 'focus', '.chzn-container input', -> $(this).parents('.chzn-container').addClass('focused')
$(document).on 'blur',  '.chzn-container input', -> $(this).parents('.chzn-container').removeClass('focused')
```
However i noticed that chosen messes up the tabindex-attributes after clicking the dropdown. this pull-request keeps the original tabindex-values, after closing the chosen-dropdown it reinserts the original tabindex.