Update log4j-core test README.
Formatting and additions.

  - Info about YAML support.
  - Example use of InitialLoggerContext for testing.
  - Fix invalid Velocity variables and macros.
  - Update Jackson version in example to 2.4.2.
  - XML casing.
  - <appender-ref/> -> <AppenderRef/>
  - Fix StatusLogger.registerListener() example.
  - Add BundleContextSelector to Log4jContextSelector table.