Merge pull request #11 from bvssvni/master

Setup library
Merge pull request #12 from bvssvni/master

Set default to rlib
Merge pull request #13 from bvssvni/master

Changed doc description
Merge pull request #14 from bvssvni/master

Added `VersionRange`
Merge pull request #15 from bvssvni/master

Added OpenGL struct
Merge pull request #17 from bvssvni/master

Added struct for GLSL
Merge pull request #18 from bvssvni/master

Added method that gets GLSL version from OpenGL
Added maintainers
Added `get_major_minor` method

Closes https://github.com/PistonDevelopers/shader_version/issues/21