Merge pull request #21 from bvssvni/master

Added gfx-rs support
Merge pull request #22 from bvssvni/master

Updated to latest gfx-rs
Merge pull request #24 from bvssvni/master

Implemented `GameWindow::get_draw_size`
Upgraded to latest Piston

Closes https://github.com/PistonDevelopers/glfw_game_window/issues/25