Add basic dot file output

This pull request is an initial solution for issue #6, visualizing graphs.  It only produces dot format files, which can then be passed to dot/graphviz externally.  Not exactly saying plot(g) and having a nice interactive window pop up, but still useful.

I wasn't able to get the tests to run without changing require("Graphs") to require("src/Graphs").  I think my local Pkg-added Graphs package was overriding the local copy.  I will take that out if it is wrong.