Promise ======= A promises utility to be used by any utility that performs asynchronous operations. This implementation is compliant with the [Promises A+ spec](http://promises-aplus.github.com/promises-spec), a standard written by members of the JavaScript community. Create promises by calling the Y.Promise constructor. Inherit from the Y.Promise constructor to return customized promises that have extra methods. The `then` method returns a promise based on its constructor, so extended promises will return the right promise type from `then`.