Adding back SCM :none

We can't use :svn because our svn server is behind the company firewall.
We can't use :rsync because that module doesn't support subversion.
Unfortunately if you don't plug capistrano with *something*, you can't even use it for basic tasks.
So this is a dummy plug to allow specifying :none as an SCM which will then do nothing.
Responsibility of getting the files up to the release_path is in your own hands.