Add empty constructor for Core::RenderCommand
Framework::IRenderable: Add methods get_{mesh,material}, rename render -> custom_render
Framework::MaterialManager: Add methods contains, get
Framework::MeshManager: Add methods contains(), get() and add()
Framework::Material: add shader uniforms when constructed
Add Core::RenderCommand to Framework::Mesh
Framework::Renderable: add properties material_name and mesh_name
Make Framework::Renderable implement revised Framework::IRenderable
Make Framework::RenderSystem's render method somewhat work
Update tests