Implemented custom event dispatching system for Piwik (replaced event dispatcher lib in libs/Event).

Notes:
  - New dispatcher can execute callbacks before or after other callbacks.
  - It is also possible to dispatch events only to a specific set of plugins instead of all plugins.
  - Moved Piwik::unprefixClass to Piwik_Common::unprefixClass
  - Added visibility to some event handlers that were missing it.
  - Allowed two unit tests to fail w/ better diagnostic messages.