Do not check urls for validity if they do not start with http. Fixes #27

Also, wrap call to requests.get in try so a proper failure can occur
That is true, but I still think this is the best way to handle the issue. I don't like the idea of a whitelist that we'd need to maintain. We have had issues on other projects with trying to check invalid protocols (such as `ftp` and `irc`) which `requests` does not support, so I think this is the best way to address that issue. I would expect major issues like typos in an href to be caught by someone before our test even gets to it.