Made the grids API object oriented. Made the forced grids functionality
work the way it's supposed to. The forced_grids keyword should be a list
of grids where None represents doing grid determination. You can now do
grid determination and force grids in the same call. Made grid
determination use polygons so it's much faster. Got rid of the shapes
configuration file and PROJ.4 grid corners are calculated on the fly.
Wrote documentation for this new grid behavior (at least for the polygon
stuff). The new main grid access point is the Cartographer class.