Add XMLResponse type for Appframework.

Similar to JSONResponse, XMLResponse allows to return a XML formatted response from App framework controller, if format is correctly specified.
@Raydiation
@LukasReschke Nooooo ... keep me in :D ... just kidding. Remove Bernhard and me. 

> Just your name is enough ;)

Handing a mail is always nice to maybe contact someone. I would leave it in there