* Use placeholder in translation string

* Fix the system test

Allows to use regular expressions in the exclude parameters option.
Doesn't break compatibility if the parameter is not a regular expression, doing a strict compare.
"/.*/" can be used to exclude all parameters.

* Changed the UI texts to explain that the excluding parameters list supports regular expressions.
Added unit tests.

Fix TrackerTest.php (includes fix to RequestCommand due to cache use in UrlHelper::getArrayFromQueryString).

Fixes  https://github.com/piwik/piwik/pull/6652

fixes #5851 - allow empty keyword parameter for all search engines, as more and more are starting to hide the keywords in referrer

Piwik is a free/libre analytics platform. Refs #4455 GNU Package requirement to use free/libre instead of open source

refs #4609

 * 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)