Remove preg_quote() call from CI_User_agent::_set_browser() and add another pattern for Opera

Input comes from a configuration file that is barely touched by anyone
and the default values only contain letters, so it is safe to not
quote them.

This enables us to add a more advanced pattern in config/user_agents.php
for Opera 10+, which ... quote:

  Opera/9.80 is hard coded at the beginning of the user agent string because of broken browser sniffing scripts which detect 'Opera/10' and above as Opera 1.

(reference: http://my.opera.com/community/openweb/idopera/)

Instead, latests versions of Opera append ' Version/<version number>'
to the end of the user agent string.

Fixes issue #555 (incorrect browser detection for Opera)
diff --git a/application/config/user_agents.php b/application/config/user_agents.php
index e555d77..2af70bf 100644
--- a/application/config/user_agents.php
+++ b/application/config/user_agents.php
@@ -85,6 +85,8 @@
 	'OPR'			=> 'Opera',
 	'Flock'			=> 'Flock',
 	'Chrome'		=> 'Chrome',
+	// Opera 10+ always reports Opera/9.80 and appends Version/<real version> to the user agent string
+	'Opera.*?Version'	=> 'Opera',
 	'Opera'			=> 'Opera',
 	'MSIE'			=> 'Internet Explorer',
 	'Internet Explorer'	=> 'Internet Explorer',