Automatic placement of nodes

Hello! Recently stumbled upon this lib, and I'm a big fan of the aesthetics -- the only big thing that's missing for me is the default assignment of node positions in a graph.

An example use case: I'm writing a pgm-type module that could make use of `daft` for nice vizualisations, but I wont know the layout of the network until the runtime of any future users (and it feels a bit mean to make them specify the layout for their model on a grid...)

There are plenty of algorithms to do this, [e.g. these summarized by Isabel F. Cruz and Roberto Tamassia](ftp://128.148.32.111/pub/papers/compgeo/draw-tutorial-1.pdf), but I wasn't sure if this was something on the roadmap or within the scope of the library. I can't instantly offer to try and implement something myself due to other priorities, but I may give it a go one day if I find time.

(oh, and if I'm being *really* stupid and this is already somewhere in the lib, apologies...)