Grapher draws the field

Sexy d3 stuff to make the grapher draw the football field. Woo! @kmdurand make sure I'm correct about the number of steps on the field: as I recall it's 160x84 steps. Thanks!

![screen shot 2014-08-08 at 7 16 16 pm](https://cloud.githubusercontent.com/assets/1168853/3867105/2386d206-1ff0-11e4-8440-a96226952a8f.png)