Upgrade to D3 3.2.0, have gem mirror version of D3

Since its much easier for developers to keep track of their JS
dependencies if the gem mirrors the version of the JS library is
gemifies, update the D3 to 3.2.0 and bump the Gem verison to 3.2.0 as
well.
Alias d3.js to d3.v3.js instead of the other way
Merge pull request #15 from thedelchop/master

Update D3 to 3.2.0 and have gem mirror the version of D3.