Add a new option for file transport to handle the log file more flexibly.

Add a 'fileDroppingHandler' option for the file transport. When the number of the log files exeeds the maxFiles option, the earliest log file would be deleted by default, if the user wants to handle the file by himself, specify this handler and do whatever he/she likes with the file(may be moving the file into another folder or uploading it to amazon S3 or anything).