If Spring Security OAuth2 is detected on the classpath, automatically configure it

* Automatically spin up Authorization Server and Resource Server
* Automatically configures method level security included OAuth2Expression handler
* Wrote extensive unit tests verifying default behavior as well as the auto-configuration backing off when custom Authorization/Resource servers are included
* Created org.springframework.boot.security.oauth2 subpackage to contain it
* TODO: Get unit test working where OAuth2-secured endpoint is properly secured with method-level security. Right now, the authorization token doesn't work.