Changes default logger used by blib2to3 Driver (#732)

... to stop it from spamming the log when black is used as a library in another
    python application.

When used indirectly by black the logger initiated in `driver.py` will emit
thousands of debug messages making the debug level of the root logger virtually
useless. By getting a named logger instead the verbosity of logging from this
module can easily be controlled by setting its log level.

Fixes #715