Fixed problem where clients could not connect in PASV mode.

Hello alanszlosek,

Thanks for implementing this library! =)

I found that there was a problem with clients connecting in PASV mode, due to a missing ```host``` variable.

I have added the fix to get the first available external IPv4 address to send to the client in the Passive Mode command.

Also, I have added support for an ```options``` object to be passed, so that the initial current working directory of the server can be overridden.