Make building work with Mingw

Applied changes by @imbaczek
Improve documentation for `Window` dropping and silence warning
Merge pull request #171 from bjz/docs

Improve documentation for `Window` dropping and silence warning
Use raw-pointer for returning GL proc address
Fix formatting
Update to latest rustc
Merge pull request #172 from bjz/glproc

Use raw-pointer for returning GL proc address
Merge pull request #170 from bvssvni/master

Make building work with Mingw