update easybiblabs/oauth2-client-php to 0.3.1
add support for StateStore in testing

This is an easybiblabs/oauth2-client-php feature implemented in 0.3.0 of
that library. It was never updated here in api-client-php
fix docblock
Merge pull request #8 from easybiblabs/topics/update-oauth2-client

update easybiblabs/oauth2-client-php to 0.3.1