Implemented Day/Night Skyboxes

Note: This build has been tested, and is deemed stable

-Added a new Material: Night skybox
-Added a new class: SkyBoxManager, which will swap day/night versions of
the map.
-Camera now holds the current skybox.
-Modified TileManager to add the map name to SkyBoxManager.
-Modified MasterController to create and store an instance of
SkyBoxManager