Upgrade D3 to v3.1.5 and add conditional comment to restrict D3 to IE>=9

This pull request upgrades the D3 library to the latest version at time of writing, expands the readme documentation a little, and adds a conditional comment to restrict IE 8 or lower from loading D3.  Without this conditional comment, even simply loading the D3 JS produces errors as per https://github.com/mbostock/d3/issues/619 - thus breaking the rest of JavaScript on a given page.

If you're happy to accept this, then could you please make a new release of js.d3?  Otherwise, I'm happy to help maintain this package.