Mobile layout coordinate system
Fix tooltips on mobile

Previously they were being cut off on the right side
Update README
Fix some minor syntax issues
IE 8 fallback to screenshot

The <a href="https://github.com/mhemesath/r2d3/">r2d3.js</a> library
_almost_ worked perfectly, but the labels were not showing up (tried
using x and y instead of dx and dy as noted in the docs but that didn't
work) and the tooltips wouldn't move for some reason (probably easier
to fix).