Refactor logging

Make use of the logging-module and kick out the custom
logging function.
Sets default values for the log-format and the log-level
to both be as verbose as possible.
Reorganize constants

Move constant-definitions further up, group definitions
and logging, improve comments
Clean LOG_FMT

Purge the format string of obsolete formatting-
instructions.
Fix wrong LOG_LVL notation

Now uses module-field instead of string
Update README

Include section on necessary preparations and
possible custom options.