Merge pull request #608 from bvssvni/master

Made game loop nicer
Merge pull request #609 from bvssvni/master

Drop reexport of gfx-rs
Added NoGameWindow, for those who want to run a windowless gameloop
Added concept of closing to NoGameWindow
Reexport NoGameWindow
Merge pull request #619 from AngryLawyer/master

Implement NoGameWindow
Updated links in README

* Replaced “Partner project” with “Back-ends”
* Fixed link to online docs
* Replaced “Tutorial” with “Examples”