Add API to allow listening for file events

[hannesdejager]

This adds a notification API where users can provide listeners to events in unFTP. These were added to the Server struct:

    /// Sets an [`DataListener`](crate::notification::DataListener) that will
    /// be notified of data changes that happen in a user's session.
    pub fn notify_data(mut self, listener: Box<dyn DataListener>) -> Self

    /// Sets an [`PresenceListener`](crate::notification::PresenceListener) that will
    /// be notified of use logins and logouts
    pub fn notify_presence(mut self, listener: Box<dyn PresenceListener>) -> Self