Adding barebones pysal2matplotlib options in viz

This very simple PR adds an option in `map_XXX_shp` methods to avoid adding axes and return only the polygon collections that matplotlib requires. It also incorporates a utility method `polys2col` to convert a list of pysal.Polygon objects into a matplotlib collection, without any added action on them.

None of this modifies the original APIs but the functionality makes it simpler to have pysal interact with cartopy, if desired.