title: MVC unobtrusive validation - Custom validator that depends on a previous selection
body: <p>I have a standard address form with a number of fields. e.g.</p>
  
  <ul>
  <li>Address line 1</li>
  <li>Address line 2</li>
  <li>Country</li>
  <li>State</li>
  <li>Postal code</li>
  </ul>
  
  <p>The problem is that the "state" field should be a required field <em>only</em> when "country" is set to USA. If the country is set to anything else, then it should <em>not</em> be required.  I could handle it in the controller but I'd like to do it on the client side unobtrusively.  Any suggestions?</p>