Added XML support

Hallo

I know that you have said before that you are not focusing on XML support at the time being, but I''ll need it for one of my own projects, so i have build it anyway. Even though it's not done, was it really simple to implement, as a hash easily can be transformed into XML. The only obstacle i came across was that XML needs a root, and json does not, but that doesn't seem to bad. Rabl eg can also render json without root with the same template as it will render xml. 

I don't know if you are at all interested, but i can't see a reason not to? 

Best regards
/Johan