LOG4J2-589 CustomLevelPlugin initial version.
LOG4J2-589 Added logging, minor improvements.
LOG4J2-589 improved comments
LOG4J2-589 initial schema with outer CustomLevels container element
LOG4J2-589 updated schema without outer CustomLevels container element
LOG4J2-589 docs: user manual updates
Javadoc.
LOG4J2-589 CustomLevel elements should be configured in a containing
CustomLevels element.

* Renamed CustomLevelPlugin to CustomLevelConfig
* Added method Configuration#getCustomLevels
* Added plugin CustomLevels to act as the CustomLevel container
* Updated manual page
* Updated schema
Install the sources jars with the main jars.
LOG4J2-861 Documentation: fix broken links on left navigation Extending
Log4j Configuration sub-menu.
Use "Documentation" prefix for all user manual/site changes.
Fix LOG4J2-862.
Move Java Lookup docs before JNDI Lookup docs.
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/logging-log4j2.git
Add Java Lookup to nav menu.
Merge remote-tracking branch 'origin/LOG4J2-589'
LOG4J2-589 added custom level configuration entry to changes.xml for
release notes
The declared exception ClassNotFoundException is not actually thrown by
the private method readObject(ObjectInputStream) from type
MessageFormatMessage.
Add missing @Overrides (except for special Java 6 JDBC case).
LOG4J2-862 added entry to change log for release notes