Add support for lftp

This pull request adds support for `lftp`. `lftp` does upload only changed files, thus is usually faster than the normal *ftp* method, which reuploads everything.

I had to move the initialization of the default port for *rsync*, please check if things are still working.
Also, I did not test anything but plain simple *ftp* (deploy.protocol = :ftp) with username and password, but *sftp* (provided the installed version of `lftp` supports it, should work too).

I wish there was a test suit that I could run, but apparently there is none.
So for now, I marked this as 'experimental' in the README and encourage others to test this further.