add client config
add token request
add client credentials session
move test property to private
update README
cleanup
move test file
extract test resource request into its own class
Fix ServerConfig

* Validation was incorrectly skipped
* Separated into two classes:
  * Generic server config for most grant types, with only token endpoint
  * Special server config for authorization code grant, with both token
    and authorization endpoints
clean up docblocks
Merge pull request #6 from easybiblabs/topics/client-credentials-grant

Add support for Client Credentials grant type