(feat) Add file-serving to Response.

Do not merge this until #48 has been merged. It will need rebasing.
This exposes the trait through `iron::SendFile`, which may or not be desirable.

The trait must be `use`d in order for its `fn` to be called, but it can be exposed through either `iron::SendFile` or `iron::response::SendFile`. If we go with the latter (without this change), it will be the only thing remaining in `iron::response`.