Add jQuery.ajax test to verify a bug.

The definition does not leverage the returned value of $.ajax()
method well. The official syntax is

```
jqXHR.then(function( data, textStatus, jqXHR ) {}, function( jqXHR, textStatus, errorThrown ) {});
```

(From: http://api.jquery.com/jQuery.ajax/)
Remove tailing space from jquery-test file
Write defition for JQueryXHR.then method.

Working on borisyankov/DefinitelyTyped#2314
Add specs for jqXHR object returned by $.ajax

The tests are corresponding to the syntax listed in its website:
http://api.jquery.com/jQuery.ajax/#jqXHR
Merge remote-tracking branch 'borisyankov/master' into bug-jquery-ajax

Conflicts:
	jquery/jquery.d.ts
Fix a asterisk bug.
Merge pull request #2348 from lijunle/bug-jquery-ajax

Write spec and fix for a jQuery.ajax bug