AMD/RequireJS support

I'm not sure if this is 100% ready, but I'm posting it as a straw-man PR.

I've brought in AMD-ized d3, dc, and dcplot (crossfilter maintainers not responding), and messed with a bunch of other stuff to make it AMD-compatible.  

It works.  It is probably not done... we could be loading dc much later but I'm not sure how to do that.

This is with dc.js 1.6.1 because (surprise) 2.0.0 has more serious problems.