Add large icons option

Added:
* 2x png sprite sheet generated from the svg
* `[largeTurnIcons](https://github.com/mapbox/mapbox-directions.js/compare/large-icon?expand=1#diff-eacf331f0ffc35d4b482f1d15a887d3bR172)` option for instruction side panel function

![](https://i.cloudup.com/_HmO6aJhJe-2000x2000.png)
> This demo has demo-specific-css, this is why the icons seem out of place

ref #37 

cc @samanpwbb @jfirebaugh