Fixing the regex in mentions.coffee

Thank you very much for kandan, it's excellent. I've been messing around with it today, and it bugged me how the mentions plugin highlighted the middle of email addresses:

![before](https://f.cloud.github.com/assets/578458/1799066/810d04a8-6b83-11e3-8c98-249a79d719f8.png)

I changed the regex in mentions.coffee to fix this, and updated the comments a little bit too.

![after](https://f.cloud.github.com/assets/578458/1799067/acfbddfa-6b83-11e3-98e0-e6114ee33d3d.png)
![after2](https://f.cloud.github.com/assets/578458/1799068/ad15cada-6b83-11e3-9a01-2d9e9ed8f098.png)

I've never worked with ruby, rails, or coffeescript before, so I'm not sure if I've done everything I need to. I kept running `rake assets:clean` and couldn't find any tests that would need updating.

If there's anything else I need to do let me know!