Allow running local commands using `on(:local)`

This works as an alias for `run_locally`. The advantage of this
feature is that Capistrano can be easily configured to perform
actions on the local host or on a server, depending on user input,
without having to replace all `on` calls with `run_locally`.

For example (in Capistrano):

    if ENV['RUN_LOCALLY']
      server :local, :roles => [:app]
    else
      server "example.com", :roles => [:app]
    end

    on roles(:local) do
      execute :any_command
    end