`std::cast` to `std::mem`
Compiled with SDL2 back-end. Need more discuss on the structure.
Rename `KeyUpEvent/KeyDownEvent` to `KeyReleaseEvent/KeyPressEvent`

  fix some bugs
Temporarily remove glfw from code
Added `EnterKey` and `SpaceKey`
Added some comments
Works with GLFW back-end togother!

  For now, compiled with `rust-sdl2` and `glfw-rs` togother. Maybe add a
  configuration for one to select.
Updated `poll_event` of `GameWindowGLFW`
Merge pull request #76 from Coeuvre/sdl2-back-end

Sdl2 back end