Improved Events.one() and fixed potential bug with Ajax persistence

As it is possible to do:
```coffeescript
myModel.bind('change', -> do_something ).fetch()
```
It would be great to allow this also when calling Events.one().

I found a potential issue with the Ajax persistence layer. The current code is using $.ajax(...).done(callback) instead of $.ajax({success: callback}) and I had missing events if the response was immediate (ie. the callback was never called after the response came from the server).
It was under jQuery 1.7.1 so maybe since then it was fixed but it is safer nonetheless to provide the callbacks before the ajax call directly in the settings object.