Remove Piwik autoloader and replace composer autoloader.
Update CustomAlerts submodule and tweak text of travis.sh no group check's error message.
Remove references to old Loader class.
Remove remaining reference to core/Loader.php.
Remove TODO from index.php.
Fix bug where certain factory methods assume new operator will throw on class not found (fatal error occurs). Use class_exists to check and then throw exception.
Refactor factory methods into base Factory type for ease of testing.
Merge pull request #6024 from piwik/composer_autoloader

Switch to composer for autoloading. Add Factory base type for factory types that create classes by string ID and fix bug in those classes where exceptions were not thrown on invalid class name (instead a fatal error ocurred).