Replace root logger with module-specific one

[timurlenk07]

Logging via the root logger is a discouraged behaviour, since the end-user does not have a way to influence the level and handlers (and other details) of log messages of the specific module. See here.

In this PR, I replace the logging calls with calls on a module level logger instance.

[pezosanta]

Hi @rshipp and @george2,

Could any of you please accept this PR ASAP? This way logging level configuration of python-nut2 works as a charm.