[Mesh] Added Mesh.

  * Created Mesh class.
  * Mesh accepts a list of Vector3 as vertices.
  * Added MeshMock for tests.
[GraphicsApiAdapter] Added Mesh and Renderable.

  * Added logic to load Renderables and Meshes.
  * Made GraphicsApiAdapter a Singleton.
[Renderable] First iteration on abstracted renderable.

  * Added support for arbitrary mesh.
  * Renderable->Start adds itself and its mesh into the graphics api.
  * GraphicsApiIndex is used to identify this renderable in the graphics api.