Add asset graphs to admin dashboard

This PR adds a new page which graphs the size of our static assets to the admin metrics dashboards.

![screenshot from 2014-01-02 16 51 47](https://f.cloud.github.com/assets/80089/1833732/3f70901a-73cf-11e3-93b4-ecb2496b6b1d.png)

#### Todo
- Use [grunt-css-metrics task](https://github.com/phamann/grunt-css-metrics) to gather additional CSS metrics
- Use [Esprima](https://github.com/ariya/esprima) to gather additional JS metrics
- Make it look pretty like GitHubs one: https://speakerdeck.com/bleikamp/sass-at-github?slide=26

/cc @rich-nguyen