Correct area calculation

This finally resolves the problems with calculating the area of concave polygons, such that we no longer have to project to 2D -- the area is now calculated directly using the standard Girard's theorem (with some cleverness about vertex ordering to get around the problems with concave polygons).  This should resolve #44 and make the areas much more accurate in general.

Also adds a fast C implementation to calculate areas, which is now the hot spot when calculating area.

@cdeil, @perrygreenfield, @bernie-simon 
OK ... it's clear from the formula below that it will be `steradian`, but still explicitly mentioning the unit of the returned area is helpful, I think.