Add FTP fetch handler

[mikedarcy]

[edeutsch]

Hi Mike, would it be possible to prioritize this? We would really like to have ftp URLs in fetch.txt. I thought this worked, but just discovered that it doesn't.

[mikedarcy]

I'm working on implementing this now. No ETA yet.

[mikedarcy]

Implemented with unit test passing as of 304f04c.

Closing issue. Please report any bugs in a separate issue.