Re-wrote logging priority levels. They are now treated as n-bit integers so you can define e.g. AgaviLogger::ALL ^ AgaviLogger::DEBUG to log everything but debug messages, or AgaviLogger::INFO | AgaviLogger::WARN to log only info and warning messages. AgaviLogger::ALL is defined as 2^32-1 so you can define level constants from 1 to 2^31. NOTE! This will break backwards compatibility! Sorry about that. closes #296.