improved named polygon shapes

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@24 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added support for "polygon" shape, with sides=4

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@25 edb9625f-4e0d-4859-8d74-9fd3b1da38cb