Add RSS Serializer

This is kind of an RFC. I could add tests, document the thing a bit better, and then I guess there are other ways this could be improved in the future - like annotations to remap object fields automatically to other RSS fields without renaming them in the json/xml outputs. 

I just don't want to spend too much time if there is no chance it will be merged, so I wanted to hear some thoughts first. This works for me as is, I give it an array in the proper format and it generates a feed.