proper logging

[NotAFile]

As it currently is everything and everybody just dumps their stuff into stdout and stderr without any indication of what the message is, what it means, where it came from and how important it is.

Ideally, the logging solution would:

• have all the standard loglevels (debug, info, warning, error, critical)
• be callable from both lua and C
• allow setting of the loglevel below which the messages get ignored with command line arguments
• allow logging to a file instead of stdout and stderr (would also fix #130)
• allow setting the loglevel on a per-module basis (so you can debug your mod while not getting engine spam)
• print source and (optionally) time of message. Time would be useful as a server admin if you are trying to correlate events, or how frequent a certain error or warning happens.