Fix logging when level is set on the logger, but no level is set on transports

Example code:
    var winston = require('winston');
    var logger = new winston.Logger({
      level: 'warn',
      transports: [new winston.transports.Console()]
    });
    logger.info('should not see this');
    logger.warn('should see this');

Expected output:
    should see this

Actual output:
    should not see this
    should see this

Reason: transport.js sets a default level of 'Info' when no level option is set on a transport.  logger.js always uses the transport level (when set) instead of using the level set on the logger.  Since a level is always set on every transport, the logger's level is never used.

I changed transport.js so that it does not set a default value for this.level.  This fixes the issue.