Add documentation to selector package-info.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/branches/LOG4J2-577@1583221 13f79535-47bb-0310-9956-ffa450edef68
Extract Status enum to its own class.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/branches/LOG4J2-577@1583222 13f79535-47bb-0310-9956-ffa450edef68