Skybox

- Basic Skybox and cube map texturing support
- Moved some functionality out from Renderer and into the client
- Cleaned up ShaderController a bit 
- Added a new shader program for Skyboxes
- Added bounding boxes to each model (useful for the next task) 
- Optimized picking by using the bounding boxes instead

- Stuff left to do: 
-> Refactoring ShaderProgram and allowing for the client to set the ShaderProgram associated with a given model (different branch)
-> Refactoring Model, Camera, and Renderer
-> Fixing player movement
-> Environment reflection and refraction (feature to add later on to Skybox)