Log levels

Proposition for log level usage:

• TRACE - Parameter values, method start etc.
• DEBUG - Intermediate results that could be useful while debugging
• INFO - Important events (e.g. new connection established)
• WARN - Something weird was detected, but it does not harm further program execution
• ERROR - A serious error occured, but program keeps running
• FATAL - Extremely serious error, the program can't continue execution

TRACE and FATAL are probably rarely used, while DEBUG and INFO are rather common.