Refactoring: use bundler/setup not Bundler.require
Configuration for Sentry, the error handling service
Add Shotgun for live reloading in development
Significant refactoring

- Remove all non-controller code from the app
- Zipper::Uploader for processing uploads
- Zipper::Password for generating passwords
- Zipper::Downloader for fetching files based on digest and filename
Split Sentry functionality into a helper