Ftplist patch

Couple of small fixes.

When using nlst on empty dirs my ftpserver (glftpd) gave a 550 err.
Added option to disable the sizegathering, to speed up process. Took quite a while for task to finish with 1000 dirs and sizegathering :)
Could you only catch the specific exception? Global except catches are not good idea, especially when they eat the exception away.