build: change `graphviz.py` to support "all"

With this change, the following:

    python tools/gyp_node.py -f dump_dependency_json
    python tools/gyp/tools/graphviz.py > node_deps.dot

.. will produce node_deps.dot with all deps.  Previously, `graphviz.dot` needed exact targets, including pathnames.

I know this is a change to upstream, just putting it here for reference.

Sample output [here](https://drive.google.com/file/d/0B1J55jsbiIIpdE5lUFVzbTdHRXc/view?usp=sharing)