refs #4439 disable eAccelerator if enabled (just disabling optimizing did not work) and display a message in the admin UI. Also disabled eAccelerator during tracking. We might need to move the method to detect eAccelerator to another place, not sure if this is the best one

(some tests failing)

renaming Unzip to Uncompress,
Renaming Tracker/Referer to Tracker/Referrer,
and few more

Removed many Piwik_ functions, in Piwik 2 it is best practise to use the methods calls instead

Todo: finish converting core/ classes + convert plugins/ classes to use \Piwik\Plugin namespace + fix build + Merge master

Refs #4059 Work in progress: Conversion to use Namespaces: Period*, Metrics, Segment, SegmentExpression, PluginsManager.
Removed some deprecated code.

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.

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