Add a parameter to the Grid to allow the default 'walkable' state to be set.

This allows the grid to be non-walkable everywhere except where setWalkableAt(x, y, true) is called.
Allow the Matrix to be used for non-walkable grids.

Update the Matrix code to flip nodes' default instead of only setting non-walkable areas. It still only mutates the nodes that need changing, though.
Updated built files.
Merge pull request #51 from Drarok/feature/set-default-walkable

Allow the default walkable state to be set, then overridden by the matrix.