Adding screenshots task

This will require to add account details to work. The expect file format for account details is a module containing an object with `user` and `key`. The file should be `ext/browserstack/account.js`.
- Run `npm install`
- Run `npm install browserstack-webdriver`
- Assumes content is under `localhost:8088` and using the `RequestrAPI` locally

There are some quirks for browsers on Browserstack that need to be ironed out.