Promise interface support

[khakulov]

amplify.request() should return jQuery.Deferred() object.

Example:

$.when(amplify.request("resource1"),
       amplify.request("resource2"))
 .then(function(data1, data2) {
           console.log("Resources are loaded!");
       },
       function() {
           console.log("Requests failed!");
       }
)```

[scottgonzalez]

We intentionally don't expose jQuery deferreds because amplify does not have a hard dependency on jQuery. You can wrap the requests in deferreds on your own if you want: https://gist.github.com/2424902