Added support for XML

RFC @doomspork
This commit adds support for XML, there are a few talking points here and possibilities for refactoring, as well as a discussion on the value this may or may not add.

Right now it will look at the Accept HTTP header, if it is application/xml or text/xml it will respond accordingly. If Accept is application/json or */* (defaults to JSON) it will return JSON.

Additional note, when working with XML we need a root node. I used `<barcode>` , If we did consider this it might be wise to think about having JSON responses with a similarly named object?

In any case, :thumbsup: or :thumbsdown: either way, here it is.