Refactor PluginManager

  - Deleted unused code from being a command line program.
  - Now use ResourceLoader.
  - Add method to load plugins from a ResourceLoader to merge with 
  the current plugin registry.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1590355 13f79535-47bb-0310-9956-ffa450edef68