Add d3 graphs for assets

- Removes sloth from http://rizzo.lonelyplanet.com/performance/css-analysis
- Adds graphs

![image](https://cloud.githubusercontent.com/assets/814861/3599268/e4c3c530-0cec-11e4-88c7-ff77e27142ef.png)

![image](https://cloud.githubusercontent.com/assets/814861/3599272/effa7fd4-0cec-11e4-8798-6e93cd9834f7.png)

You could try something like this instead of manually putting them all in (just for future reference - no biggie at all): https://github.com/lonelyplanet/rizzo/blob/master/.gitignore#L53-L54