twisted.protocols.ftp.FTP should emit audit information

[exarkun]

It should be possible to have a log of all the commands and transfers performed by an FTP instance.

One way to implement this would be to emit structured log information for each command and provide an observer which watches for this and writes it out to a file.

This might also make at least some of the log.err calls in FTP unnecessary, since there would be another place to look to find the information. This mostly applies to cases where FTPCmdError failures are logged, since those have an obvious translation to a log (since they have a response code in them).