Feature/notify improved

now:

* `dw.backend.notify()` returns an object with `remove()` and `get()` functions. Caller can now remove the notification after showing it.
* this object is bound with the notification element as `_controller`.  
Then, you have to remove the notification always with the `remove()` method, either by the object returned during creation or by the element via the `_controller` parameter. (For example, to remove all the notifications : `$("#notifications .notification").each(function(){this._controller.remove();})`)
* The message "Watch out: Your chart needs more space. To get rid of the scrollbars just resize iFrame to chart" is now using this new method to automatically remove the notification if the iframe size is enough.
* All the notifications are removed after each visualization change. (Maybe some notifications need to be persistent?) da7a042