WIP on running djax unit tests with jQuery.ajax

The goal is to check that djax actually is a subset of jQuery.ajax.
The latest problems I still have are:
 1. I cannot run properly `gulp test`, since the server mockup apparently does not restart at the end of the first tests suit.
 2. There are two weird things with running the tests with jQuery (search for "TODO" in djax.test.js)
Fixed issue with jqXHR#then

I misunderstood how exactly this #then method works. Djax's implementation is currently wrong.
Source:
https://gist.github.com/bloodyowl/c7e6fb0a08e69c693323
Fixed gulp test task

The API mockup is now loaded two different times for both jQuery and djax (I just needed to clear require's cache).