added "square" node shape

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@26 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
added "diamond" node shape

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@27 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
better naming of node grobs

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@28 edb9625f-4e0d-4859-8d74-9fd3b1da38cb