Fixed done flag

The done flag is now set to true in every existing exits for the ajax call.
Also, the conclude function is now added in case of an aborted call due to a beforeSend returning false.
Replaced "ok" status by "success" status
Simulated JQXHR instead of just returning promise

Basically, the .then(), .done() and .fail() methods are now added to the XHR object instead of being just returned in a new object.
This means that the object returned by the ajax() function is now the exact same object that is given to callbacks as XHR.
Updated tests to take into account last 2 commits

 - Using "success" instead of "ok" as status
 - Checking that the xhr argument given to callbacks is the same object as the one returned by the ajax function instead of checking that it is an instasnce of XHR
 - Added a test to check that the .done() method works even after the call has been ended (ie when the "done" flag is true)
Added version in djax
Release v1.0.1