Upgraded to Spring Security OAuth2 2.0.2.RELEASE + Grails 2.4.3

There are some significant API changes from version 1.x to 2.x of the Spring Security OAuth2 library, including renamed and relocated classes, setters moved to constructors, dropped parameters, etc.
I also updated to the latest Spring Security Core plugin (2.0-RC4), Grails 2.4.3, and upped the minimal Grails version to 2.3. 

Please have a look!
You can `import grails.test.spock.IntegrationSpec` in Grails 2.3 >. I wanted to make sure the `springSecurityService` got injected properly for testing the secret encoding.