Refactor logging + use it for debugging messages

[madebr]

• refactor logging to a design based on the logging module of python
• use fmtlib for formatting the message
• use logging for printing (all) debugging/... messages
• minimize the overhead (or none if possible)
  ... (go crazy)

[danhedron]

Some of the debug messages are just assertions, these should remain simple, but everything else should use logging unless there's a performance reason for them not to.