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.

Refs #3124, add benchmark for MultiSites.getAll and benchmark fixture that adds 20,000 sites to the DB then tracks one pageview per site.

Fix transitions segmentation issue

HTML & CSS clean up. See below for review & regressions. 

Refactor Tracker_Visit, moving 400+ lines to a new Tracker_Request object. This class contains all getRequestVar calls and all knownledge of request params.

Cleaning the Tracker_Visit class, new Tracker_VisitExcluded class, and moved Tracker_Referer to its own file

Fixing the "testAllSizes" debug output at: index.php?module=ImageGraph&action=testAllSizes&idSite=1&period=year&date=today

automatic translation update