Merge: stream: push-back reader

I implemented something similar to http://docs.oracle.com/javase/7/docs/api/java/io/PushbackReader.html last week thinking I needed it for the XML parser, but I ended up with a top-down parser that do not need to backtrack by more than one octet.

Pull-Request: #805
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>