Adding support for Theleague oauth and new implementers for social auth and social post

Theleague is Oauth2.0 client library allows integration with any OAuth provider that conforms to the OAuth 2.0 standard, theleague provide a GenericProvider that may be used to connect to any service provider that uses Bearer token. Apart from this many providers provide additional functionality so thephpleague can be extended and wrapped for additional behavior. The objective of this project will be to integrate Theleague into Social API project and add new implementers to Social Auth and Social Post.