Move things from app.js to index.js

- Moved a few things from the app into the peerfix module:
  - `engine.on('uninterested')` behaviour
  - `server.listen` when engine is ready.

Also..

- `app.js`: Removed unnecessary `require('fs')`
- `index.js`: Commented a few lines.
- `index.js`: Renamed `engine` to `torrentStream` and `e` to `engine` for the sake of clarity.

Related https://github.com/mafintosh/peerflix/issues/62