provide a complete deploy:rollback task

This is a working proof-of-concept for #551.

Changes:
- add :cleanup_rollback to archive and remove rolled back release.
- add :finalize_rollback in rollback's runlist.
- rollback's runlist now becomes:
  ```
  %w{starting started finalize_rollback
     finalize restart cleanup_rollback finished}
  ```

Note that, `:finalize_rollback` hook is copied from #549, related discussion: capistrano/rails#3.