Consider introducing more granular log levels

Currently, mostly info and debug are used. While the former gives a reasonable amount of detail, the latter is already too much. In order to provide more readable logs, I would propose to add some additional log levels.

Proposition:

DEBUGV    (5)
DEBUG     (10)
PROGRSS   (13)
STATE     (15)
NOTE      (17)
INFO      (20)
HILIGHT   (25)
CAUTION   (29)
WARNING   (30)
ERROR     (40)
CRITICAL  (50)

It's not too difficult manually. Alternatively, the verboselog package does that for us.