Using the simplifiedAutoLink option does not return the expected GFM behaviour when parsing links without a http prefix.
Previously, `www.google.com` would be parsed into `<a href="www.google.com">www.google.com</a>`.
With this fix, showdown behaves like GFM, and the result is `<a href="http://www.google.com">www.google.com</a>`
Closes#284, closes#285
