Fixes for using refile inside an engine

Two changes to make refile work better inside isolated engines, fixes #15 

1) It replaces `refile_app_path` with `main_app.refile_app_path` which is ok because currently refile is always mounted in the main app, i.e. it uses `Rails.application.routes.draw` in `config/routes.rb`

2) It moves the attachment helpers to `lib/refile/attachment_helper.rb` and mixes it into `ActionView::Base`. This makes the helpers accessible to all engines, not only the main app. I personally like the way https://github.com/plataformatec/simple_form/ structures their files, with an `action_view_extensions` folder, where they have both the view helpers and the form builder helpers, but I didn't want to move too much stuff around. Worth considering perhaps.