Merge log4j-plugin-processor to log4j-core.

  - Updated maven-compiler-plugin settings for log4j-core so that the annotation processing works properly.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1590194 13f79535-47bb-0310-9956-ffa450edef68