v2.0.0

Added unit tests, with only a couple changes, but one of them is potentially a breaking change.

• Removed the class IntrinsicPriorityQueue. It wasn't actually used by the rest of the code, so I figured there was no point in maintaining it separately. It didn't really belong. (PathSolver does use the internal class IndexedIntrinsicPriorityQueue, but it's too specialized and too light on error checking to be a library-quality data structure on its own.)
• PathResult.PathCost is now double.PositiveInfinity instead of 0 when no path can be found.