Adding rotationFormat prop to file.js

The goal of the rotation format property is to allow the user to pass a pointer to a function that will determine the format of the file name in case of a rotating logger (a logger with maxsize that generated files). 

sample usage:
this will make the files created under "log" to be named:
run<yyyyMMddhhmmss>.log

so these would be files created today:

`run20130629190056.log`
`run20130629190057.log`
and so forth...

sample code:
```js
winston.add(winston.transports.File, { 
  filename: 'log/run.log',   
  maxsize: 150,
  rotationFormat: function() {
	return getFormattedDate();
	function getFormattedDate() {
    var temp = new Date();
    return dateStr = padStr(temp.getFullYear()) +
                  padStr(1 + temp.getMonth()) +
                  padStr(temp.getDate()) +
                  padStr(temp.getHours()) +
                  padStr(temp.getMinutes()) +
                  padStr(temp.getSeconds());    
	}
	function padStr(i) {
		return (i < 10) ? "0" + i : "" + i;
	}
  }
});
```