Add a copy of D3's license to comply with it

This is still the same version of D3 (3.2.7) as downloaded from:

  https://github.com/mbostock/d3/releases/tag/v3.2.7
Upgrade D3 to 3.4.6

This is the latest upstream version, available at this URL:

  https://github.com/mbostock/d3/releases/tag/v3.4.6
Merge pull request #563 from fmarier/upgrade_d3

Upgrade D3