Adding accessibility to chosen:

- Added unique_id which is used for ARIA attribtues
  that require an ID. So now search_field and
  search_results have an ID
- Added the required ARIA tags for chosen to behave
  like a combobox
- Tested with Mac OSX Chrome 36 using VoiceOver,
  now the screen reader is able to read the values
  in the chosen select box, like it's a SELECT
  element