Add path finding for Directed Acyclic Graphs

[roy-t]

If we add path finding for Directed Acyclic Graphs, and also add an option to convert a grid to such a graph. We can make path finding a lot more flexible as this gives more control over what tiles are reachable from another tiles.

This could fix #16 by removing links between tiles that would cause cutting a corner.
This could fix #17 by giving links between tiles a direction.

Its still unclear what the performance impact would be. Do we need both algorithms or is path finding equally fast on a DAG?

[bradzoob]

Hadn't seen you'd posted this 2 weeks ago! This would be awesome :)

[roy-t]

So this turned out to be a complex problem. Graphs are usually faster because you can use a less dense representation, but performance optimizations for graphs and grids are very different. I've decided not to implement this feature.