Add d3.geo.vanDerGrinten4.invert.
Add d3.geo.rectangularPolyconic.invert.
Improve d3.geo.[rectangular]Polyconic.invert.
Better initial estimate.
Simplify d3.geo.eisenlohr.invert.
Tweak d3.geo.eisenlohr.invert.

Slightly faster, and better handling of points near poles.
Optimise d3.geo.eisenlohr.invert.
Fix d3.geo.guyou and add inverse.
Improve d3.geo.guyou.invert.
Add d3.geo.peirceQuincuncial.invert.
Add d3.geo.healpix.invert.
Add tests for d3.geo.healpix.
Fix NaNs in d3.geo.vanDerGrinten4.

Fixes #43.
Split out quincuncial projection code.

This will be reused by Gringorten, and any other projections in future
that project a hemisphere into a square.
Fix quincuncial inverse when no inverse is defined.
Reuse quincuncial for Gringorten.
Update geo/projection/README.md
Add d3.geo.gringorten.invert.
Save a few lines.
Fix token replacement in jsonp plugin