add support for app running in a subdirectory

For example, if the Rails app lives in the "rails/" subdirectory
of the repository, then putting

  set :app_subdir, "rails/" # trailing slash is required

in config/deploy.rb will make everything work by adjusting the
paths accordingly.

See also https://github.com/capistrano/capistrano/pull/616 which
adds support for the same variable to capistrano core.