Allow dojo.io.bind to specify successful HTTP codes

[david]

Something like:

dojo.io.bind({
   acceptCodes: [404, 500],
   url: "...",
   load: function() {
      alert("The server puked, but I'd like to call that a success");
   },
   error: function() {
      alert("The server did just fine. Isn't that a shame.");
   }
});

[bill]

When we do this will probably want to pass in a function to check for error, rather than an array of return codes.