Misc changes and fixes

- Fixed an error in Matrix::createOrthographicOffCenter.
- Changed orthographic camera mode to have an origin of bottom left, with +Y up and +X to the right.
- Added configurable character spacing to the Font class.
- Changed Scene:addNode and Node:addChild to add nodes to the back of the list instead of the front, so that traversal order matches the order in which nodes are added.
- Added support for creating hierarchy (child nodes) in a .scene file.