Update dc.js to v1.6.0

Source taken from

https://raw.github.com/NickQiZhu/dc.js/1.6.0/dc.js
https://raw.github.com/NickQiZhu/dc.js/1.6.0/dc.min.js
https://raw.github.com/NickQiZhu/dc.js/1.6.0/dc.css