storage/ftp: add FTP support

[buchanae]

There's a little bit left to do here:

• auth credentials should have a place in the config file. currently the code only supports pulling user/pass from the URL.

That could be done in a future issue though.

[buchanae]

A quick update: this works and passes the tests in multiple environments I've tried, but only fails in Travis. So...yay.

[buchanae]

The TravisCI test is fixed. The test FTP server needed help matching up the user/group ID with the host.

There are a couple items left:

• add better config so that user/password aren't in the storage URL.
• I think there's an intermittent failure where two tasks create the same directory concurrently.

[buchanae]

Ready to go