Not possible to use $.ajax(...).abort()

Using with zepto (`Deferred.installInto(Zepto);`); it is not possible to cancel an in flight ajax request with `$.ajax(...).abort()` like you can in jquery because an ordinary promise is returned from $.ajax()

The attached commit fails the test cases because it returns a promise with an extra attribute (`.abort()`) -- you very well may want to handle it a different way but this was the starting point I was thinking of.