Add script/bootstrap and script/test

The `script/bootstrap` command does all the things you need when you first clone bridge_troll. It's also meant to be idempotent and not clobber anything when run after pulling new changes from upstream.

In addition to `script/bootstrap` I also added script test which makes `bundle exec rake` just happen. This is less important but still saves anyone from thinking about how to write tests if they don't feel like thinking.

Lastly, I amended the README to use script/bootstrap where appropriate.