Notes:
  - Modified ManySitesImportedLogs fixture to add segments if desired.
  - Switch to synchronous execution of curl requests until deadlocking issue in archive processing is dealt with.
  - Fixed bug in LogAggregator where if a segment required a subquery, archiving would failed.

Allow aspects of the testing environment to be init-ed by setting globals so it is not required to include of all of Piwik.

Make Piwik_Access a singleton instance and remove use of Zend_Registry for the Piwik_Access singleton.

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.

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

Refs #4019