changed arrowed length to default 10 'native'in line with graphviz

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@12 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
reverted 'gap filler' in makeArrow()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@13 edb9625f-4e0d-4859-8d74-9fd3b1da38cb