Make logging at debug levels more convenient

Previously, even though MINIMUM_LOG_LEVEL could be set, if set to a
severity like "DEBUG", log entries would be ignored anyway because of
the hard-coded setLevel calls with logging.INFO.

Instead, set the severity from MINIMUM_LOG_LEVEL so that the former
does the more obvious thing when changed to a value like DEBUG.

Reported by Greg Stark <stark@heroku.com>