Added @sensepost FTP implemenation

See this https://github.com/rapid7/metasploit-framework/pull/3229 for more info.

This PR adds the FTP implemenation with a few more checks than in PR 3229

I tried it on my test server and got some data back, but it didn't show me some "real data" even when I set the server under pressure with a lot of ftp uploads.

Maybe @wvu-r7 and @sensepost can test and verify this implementation.