Option to start new logfile every 'day' (or actually specified timespan)

[janwilmans]

This option is to allow logging to a file over a very long period. we should probably support:

• create new log file every N-days
• create new log file every N-Mb (megabyte)

For this to work, we first need proper support to log to a file at all (currently buggy) see also related #28 #109 #167 #171