Merge pull request #34 from bvssvni/master

Removed support for `ConcurrentGame`
Merge pull request #35 from bvssvni/master

Fixed long line
Merge pull request #36 from bvssvni/master

Added support for custom OpenGL version
Merge pull request #37 from bvssvni/master

Changed from `[[lib]]` to `[lib]`
Merge pull request #39 from bvssvni/master

Take OpenGL version by value
Merge pull request #40 from bvssvni/master

Upgraded to latest Piston
Merge pull request #41 from bvssvni/master

Removed dependency to shader_version
Merge pull request #43 from bvssvni/master

Made window resizable
Release captured cursor

Closes https://github.com/PistonDevelopers/sdl2_game_window/issues/44