Support retrying bind calls

[david]

Allow people to specify the number of retries a bind call can make in case there's a network error. Something like:

dojo.io.bind({
   url: "foo",
   attempts: 3,
   load: function(...) {...},
   error: function() {
      alert("I tried to submit 3 times, but it didn't work");
   }
});

Success would be defined at the HTTP level (any request that fires the "load" handler is a success).

[alex]

no one else has ever asked for this. Wontfix.