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
Updated symlink info
Updated symlink info
Update README.md
Update README.md
Updated symlink info
Moved `keycode` to `keyboard`
Moved `event` to its own module
Added `mouse` module
Added mouse support
Fix bug that `mouse_move` event not working with GLFW back-end