Timing of 'chosen:showing_dropdown' fixed

Currently the event is triggered before dropdown LI's
are created. This commit corrects it by moving the
trigger event at end of method. A JSFIDDLE was created
to demonstrate the issue.

http://jsfiddle.net/lagrz/3A5jq/

By moving the event it enables developers to add
whatever additional code or styling to the dropdown
menu options such as images.
Fire the `chosen:showing_dropdown` event only after results
are totally built.

Further information about this fix is described in:
3246598cbb93b66026828ab85754c65378f43ea9