Allow wildcard domain for 3rd party cookies to be set explicitly in config.ini.php

Pull request for the enhancements discussed in this thread: http://forum.piwik.org/read.php?2,107159,page=1#msg-107379

Please note that in order that this feature can work propperly an entry in the config.ini.php must be set. The key for that entry is "cookie_domain". I was not sure whether there must be a default value set somewhere else for that config key. Means: I do not know whether there would be a runtime error in line 418 of Request.php in case that value is not set in config.ini.php.