Add support for layouts

[cmgambino]

GraphViz has support for the following layouts: dot, neato, fdp, sfdp, twopi, circo.

Could it be possible to allow the DotGraph object to be configured with an optional layout style?

[eXpl0it3r]

Not too familiar with GraphViz, can you explain what these layouts mean exactly?

[vfrz]

Sorry for the delay, I've made a PR: #24
Is it what you want?