Sanitize shouldn't filter out URLs without protocol.

Fix for issue #81.

Sanitize was deleting links such as `[link](/page.html)` since they did not have a protocol. This doesn't seem like desired behavior.

Please note the test changes, specifically that `src=#` is no longer removed with this change. I'm not sure if that's ok or not, so another case may need to be added for that. I also added a test case for relative links and tested against my own code to verify.