When autolinking urls, make sure there is always a protocol on the href

We had to fix this autolinking anomaly after the fact in the twitter.com repo, it might be nice to fix it here instead.  For example, <a href="example.com">example.com</a> is broken without a protocol.

Also, someone may wish to make extractUrlsWithoutProtocol default to true when calling twttr.txt.autoLink soon.  Or at least allow the options to override it. Or both. Considering twitter's backend has been doing autolinking without protocol for several months now, it doesn't' make sense for this to be hard-coded off.