Display logging levels in messages by their names, not numbers

[Leden]

This is bad:

ts=2015-06-11 13:54:35.222713 level=20 guid=d045b00c-8980-4a28-a33a-0a5bb455e8c9 event=make_audience.start args=[] kwargs={}

This might be better:

ts=2015-06-11 13:54:35.222713 level=INFO guid=d045b00c-8980-4a28-a33a-0a5bb455e8c9 event=make_audience.start args=[] kwargs={}