Fix 2 errors caused by recent commits
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)
Add CI_User_agent::parse() to allow parsing a custom user-agent string

Based on PR #970
Merge branch 'develop' into feature/form_validation/custom_error_per_field
Added the missing "is" in the comment
Changed the rest of 'error_msg' to 'errors'
Added space after 'if' condition
Fixed coding styles in form_validation.rst