Upgrade to grails 2.3.4
-Resolve issues with missing jul-to-slf4j dependency
-Upgrade to release plugin 3.0.1
-Add definition for performInjectionOnAnnotatedClass, missing from Slf4jTransformer
Apply changes to allow an external configuration file to take precedence over any internally defined logback definitions for GPLOGBACK-6.

Note: the changes currently in place only resolve things at runtime, there is still problems under run-app.