Add package to provide an OAuth client

Hi, 

Since in the future Icinga Web might want to support OAuth, or modules needs to integrate with it, it would be nice to have a package in the icinga-php-thirdparty

For example: https://github.com/thephpleague/oauth2-client

* Generic Client that can be extended 
* Supports a wide range of PHP versions
* Minimal transitive dependencies  (ext-json, guzzlehttp/guzzle)
* MIT Licence 