(feat) Makes response.serve() take a BytesContainer

This means we no longer have to explicitly call .as_slice()
all over the place, because both String and &str impl
BytesContainer. See: http://doc.rust-lang.org/std/path/trait.BytesContainer.html
Merge pull request #89 from reem/feat/bytescontainer

(feat) Makes response.serve() take a BytesContainer.