Add client-complete-pending-command method.

For certain FTP commands it's necessary to do more work to complete a transaction. In my case I was GET-ing multiple files in the same session using `client-get-stream`. The second call to `client-get-stream` returns a nil stream and a reply code of `226 - TRANSFER COMPLETED`. This is the final response from the previous FTP command. This should have been retrieved with a further command before proceeding to the next GET. There is no way to do this currently in clj-ftp.

This Pull Request adds a wrapper around `client-complete-pending command` and
a test proving that it's needed when downloading multiple files. I suspect other FTP commands should use this too, but I haven't come across them (yet)

See [Here for details](http://commons.apache.org/proper/commons-net/apidocs/org/apache/commons/net/ftp/FTPClient.html#completePendingCommand%28%29)