added pkg/R/grobs.R with nodeBox() function to handle different node shapes in node()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@45 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
moved makeLabel() to grobs.R

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@46 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
moved makeArrow() to grobs.R

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@47 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
renamed: makeArrow() to makeArrowGrob(); makeLabel() to makeLabelGrob(); nodeBox() to makeBoxGrob()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@48 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
documented box and label grobs in node()

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/gridgraph@49 edb9625f-4e0d-4859-8d74-9fd3b1da38cb