Add an at-symbol to provide hinting at desired contents.

While it doesn't matter because Twitter can handle an at-symbol in the URL, it's not really desirable to expose at-symbols in the URL, especially since they're being emailed to users.

Also, it looks cool. :)

![twitter-symbol](https://f.cloud.github.com/assets/42032/1104648/5e55d768-1904-11e3-8a62-d01485d0f24d.jpg)