Reworked error handlers

* Errors throw an ErrorException
* Exception handler takes care of logging and displaying the error page
Added log severities CRITICAL and ALERT
Merged function ww_setting() into System::setting()
Make use of Util::toTimestamp() in method Http::setCookie()
Moved function theme() to class Util
Moved function parse_settings() to class Util
Fixed bugs and missing documentation in Util class
Removed method Http::setHeader()
Merged class Http into class Util