Adds backend for MineID.org

[MineID.org](https://www.mineid.org/) is a free and Open Source OAuth2 Provider for Minecraft. It works as a wrapper for the Mojang's authentication service. The source is available at http://github.com/MineID/MineID

This pull-request contains the backend, tests, documentation and changes in the project examples.

Let me know if you need anything else.

Thanks