title: Fill select dropdown based on selected item
body: <p>I'm working on a rails app.</p>
  
  <p>I have a dropdown with countries.  Selecting a country would fill the city dropdown with the cities in that country.</p>
  
  <p>I know how to do this server-side, but I'd like to do it client-side if possible.  I've used the RJS helpers, but I've no experience with jQuery (not <em>exactly</em> even sure what it is) or the other selector javascript libraries.</p>
  
  <p>Clarification: I'd like to do this <strong>entirely</strong> client-side. That is, send all the values in the initial page request. I'm not looking to call the server ajaxically, hit the database, then return those results. I'd like them loaded up with the page.</p>