Cahill keyes

This is to contribute a new map projection: [Cahill-Keyes](http://www.genekeyes.com/MENUS/C-K-linklist.html).
I have a working [example](http://bl.ocks.org/espinielli/5689783) which uses d3.js version 3.1.8 but which fails with latest version of d3, 3.2.1, because it lacks d3.projection.clipPolygon.

I organized the things as follows:

1. defined d3.geo.projection.cahillKeyes coordinates transformation in geo/projection
2. added d3.geo.polyhedron.cahilKeyes in geo/polyhedron

The way I was able to have all working was by taking Airocean example from Jason, understand it, copy the polyhedron.js code from there and add the relevant cahillKeyes part.
So in creating my contribution to polyhedron.js I have tried to merge from there all that seemed to make sense:
* line 3:   var ε = 1e-5, // 1e-6 gives incorrect clipping for cahillKeyes
* line 109 use clipPolygon (and later mesh):  var projection = d3.geo.projection(forward).clipPolygon([clipPolygon]);
* line 112: I had to comment out the projection.stream code
* I added a small test, more coordinate transformation should be added. I will try to get the numbers from the perl code from Gene Keyes's web site as a reference implementation later.

There is some other work to add an inset for Antartica, but I see it as a further step for a more specialized map...

Feedback is welcome (and anyway thanks for the fantastic work you have done!)