SGF parser

I played around a bit with creating an [SGF](http://www.red-bean.com/sgf/) parser. I haven't gotten very far yet, but I still would like to have your feedback. I'm especially interested in the life time stuff. Why do I need it for `Parser::new` but not for `Parser::board`? And am I using string correctly? I had a bit of a problem getting it to compile (I tried `str`, `&str`, and `~str` as types for the `sgf` field in `Parser`).