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
Add JSDoc for JQueryXHR.abort method.

Inspired the description from its source code:
https://github.com/jquery/jquery/blob/d41e81980ea89e56c04eb14d8c27f4692b367604/src/ajax.js#L487
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
Merge pull request #2316 from lijunle/jquery-xhr-doc

Add JSDoc for JQueryXHR.abort method.