Add a full-stack integration test suite

We've had several bugs that were only reproducible when Curly was integrated into an actual Rails app. In order to make debugging these problems easier, and to prevent regressions, a full-stack test suite is introduced.

The specs exercise a minimal Rails app that uses Curly in the view layer.

###### Tasks

- [x] Add an integration spec suite
- [x] Make the integration specs play well with the normal specs
- [x] Cover the use of Rails helpers
- [x] Cover the use of Rails partials