[Roxygen-devel] Callgraph roclet

[Peter Danenberg]

The callgraph roclet is in with @callGraph, @callGraphPrimitives and
@callGraphDepth; a couple things to note:

    * it introduces an Rgraphviz dependency into the main tree;

    * since it relies on `eval', it requires dependencies to be
      installed beforehand; including the package itself.

It may require running roxygen, therefore, installing the package and
running roxygen again to get the callgraphs; like running LaTeX twice
to get the references.

`pkg/sandbox/roxygenize.R' now generates a couple graphs as
proof-of-concept.