Rewrite expressWinston#errorLogger and expressWinston#logger to use winston.log

The idea here is that I wanted to be able to do something like this:

    var winston = require('winston');
    var expressWinston = require('express-winston');
    
    winston.add(winston.transports.File, { filename: 'output.txt' });
    ....
    app.use(expressWinston());

...and have expressWinston use winston.log to log to the transports added using winston.add.

There was an additional change associated with this. I replaced the loop which logged to each of the transports with the creation of a winston logger (`var logger = new winston.Logger({transports: transports})`) and a single call to `logger.log`. I think this is a little bit tidier.

Thanks for the great module guys!