Implement handling of options that are both disabled AND selected

[dingram]

Allows Chosen to deal with items that are both disabled and selected (i.e. should show as selected but should not be deselectable). See also #516 and #586.

These changes include:

• Different class/styling on the disabled item
• Removal of the "x" link on the disabled item
• Tweaks to backspace-handling, to ignore disabled items
• An inadvertent fix for the Prototype code (see below)
• Update to the example files

This change does not update the compiled files, as I appear to have a different version of coffeescript (or different options), and didn't want to create needless changeset noise. This can be updated, however.

Prototype fix

I noticed that in keydown_backstroke (around line 490), it was calling:

@pending_backstroke = @search_container.siblings("li.search-choice").last()

but Prototype does not accept a selector argument to siblings(), so this was doing nothing. In order to replicate the functionality from jQuery, it would have to be something like:

@pending_backstroke = @search_container.siblings().accept( (x) -> x.x.hasClassName("search-choice") ).last()

with an additional tag name check. In any event, I updated the code to use:

@pending_backstroke = @search_container.siblings().reject( (x) -> x.hasClassName("search-choice-disabled") ).last()

as I needed to exclude disabled items, and it appeared to be doing the right thing anyway.

[dingram]

As noted in the main description, the selector "li.search-choice" has no effect here. In fact, the siblings() method accepts no arguments at all.

[ValterBorges]

#586 had a bug. here is the fix. sry new to github not sure how to make a formal pull request.

//add if !== undefined

  if(result_data !== undefined)
  {
    result_data.selected = false;
    this.form_field.options[result_data.options_index].selected = false;
  }

[pfiller]

Thanks @dingram. I've created a new pull request (#638) that builds on this PR. Would welcome feedback there and hope to merge to master soon.