Support Path Finding in Arbitrary Graphs

This adds support for path finding in an arbitrary (i.e. non-grid) graph. This graph is represented by `MOAIVecPathGraph` and can be used with `MOAIPathFinder`. A sample demonstrates the new functionality.

I had to tweak the algorithm in `MOAIPathFinder` a little to make sure it finds the shortest path, not just any path. The change is to take the cost of the path so far into account when finding the best path to pursue. If I'm not mistaken, this is now a complete implementation of A*. See also moai/moai-dev#594, which can be closed when this gets merged.

I have updated the project files for OS X, iOS and NaCl. Android and CMake seem to use globbing and should work out of the box. Could anyone add MOAIVecPathGraph.cpp/.h to the Visual Studio projects after merging?