xkcd: added Explainxkcd.com link, updated JS style

Fixes #21.
![screenshot - 07212014 - 08 22 50 pm](https://cloud.githubusercontent.com/assets/4411471/3652230/55255b70-1136-11e4-9e32-a23fff182097.png)
I also added a closure and fixed the code style of `xkcd_display.js`.