assets:sync:clean task

- Expose asset delete as`assets:sync:clean` task
- `assets:sync` and `assets:sync:clean` now call `#invoke_or_reboot_rake_task` from `action_pack assets.rake` so that you don't need to add `RAILS_GROUPS=assets` when running them (assuming your assets_sync gem is in the `:assets` group)
- Add Rubymine to .gitignore