xkcd: added Explainxkcd.com link, updated JS style

Fixes #21.
![screenshot - 07212014 - 09 08 18 pm](https://cloud.githubusercontent.com/assets/4411471/3652483/b26722ea-113c-11e4-84bc-499abf9fe949.png)
I also added a closure and fixed the code style of `xkcd_display.js`.