Use named loggers

[cunha]

The library currently uses the root logger, which prevents
applications from separating their logs from the library's.

With this change, applications can silence or redirect sagan
logs by reconfiguring the "ripe" logger. In particular, the
following will print only FATAL messages from sagan:

def main():
    ripe_logger = logging.getLogger("ripe")
    ripe_logger.setLevel(logging.FATAL)
    ...

close #89