Graphviz is a real shepherd dependency, not a devDependency

Hello anybody, 

Please review the following commits I made in branch 'nick-graphviz'.

7ed004a0f647d4a1013179039c723af42ffdc84b (2013-07-22 10:46:26 -0400)
Graphviz is a real shepherd dependency, not a devDependency


R=anybody