Refs #4125 Renaming plugin from Referers to Referrers

 * updating all variables/methods containing Referers
 * RENAMING custom segments definition, and custom scheduled reports definition, so that when they load "Referers" segment/report it still works as expected
 * RENAMING old archived data so that archived blobs are called Referrers
  * Reverting part of 0deb2bbb
  * Update integration tests generating PDF+HTML with images on my dev box (GD 2.1 and PHP 5.5)