Add engine attribute to Graph, and default to 'dot' layout

[abelsiqueira]

On #38, we noticed that some position-related attributes are not working (e.g., rankdir). Apparently, the code is using the layout "neato", and the layout that we want to use is "dot".

I've added a field to Graph to allow changing the engine. I don't know the details on GraphViz, so I am not sure how adequate the implementation is. I also changed the default to "dot", but I can change it if desired.

I have created a test and an example on the README.

[moll-dev]

Hey! I just ran into this issue today. Did you happen to push your changes to a package repo?

[abelsiqueira]

Hi @QuantumFractal, not to anything registered, but you can install my branch using

pkg> add https://github.com/abelsiqueira/GraphViz.jl#38-add-engine-attribute

[Keno]

It seems odd to make this a property of the graph itself, rather than an aspect of the display process, but GraphViz itself is somewhat blurry about this, so that's fine I guess.