Added SurfaceTile. Fixed minor problems in a few other classes.

git-svn-id: http://worldwind31.arc.nasa.gov/svn/trunk@2517 822214bb-dcf7-4c8d-a767-17ae0fb20d88
Filled out stubs for TerrainTile. Added NotYetImplementedError and used it in several classes. Documented Tile.

git-svn-id: http://worldwind31.arc.nasa.gov/svn/trunk@2518 822214bb-dcf7-4c8d-a767-17ae0fb20d88
Moved GpuProgram to the new shaders directory.

git-svn-id: http://worldwind31.arc.nasa.gov/svn/trunk@2519 822214bb-dcf7-4c8d-a767-17ae0fb20d88