Add basic OSGi activator class.

  - Registers Log4jContextFactory as the LogManager.factory.
  - Tracks future bundle start-ups to find Log4j plugins (still need 
  to add code to check existing bundles) via the plugin cache file.
  - Pre-loads plugin classes using proper ClassLoader for future use 
  by configurations.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1590443 13f79535-47bb-0310-9956-ffa450edef68