Add 404 and 500 error pages

I've added some default 404 and 500 error pages. Looks like this:

![capture decran 2014-01-17 a 18 16 52](https://f.cloud.github.com/assets/103900/1942655/eaf9b8ca-7f9c-11e3-918f-cd29c5fd6f93.png)