Dummy test app

[pablobm]

When I merged #18, @nickcharlton threw the question of how we could go about testing that. This PR attempts to provide an answer.

Here I set up an dummy app, same as the ones used for engines, as well as feature specs that run on this dummy app.

There are a bunch of files because setting this up is not trivial. In summary, I added:

1. A dummy app at spec/dummy. I copied it from a newly-created engine project and adapted it to work here.
2. In the dummy app I added in turn:
   • A couple of models, ensuring a has_many relationship.
   • An admin section with administrate, using administrate-field-nested_has_many.
3. A feature spec for the problem at hand.
4. Appraisal, to allow testing the dummy app against different versions of Administrate.

To run the tests: bundle exec appraisal rake. Should document this and set it up on CI.

To run the dummy app: rails s.

On an implementation note, there's too much logic in the template at app/views/fields/nested_has_many/_show.html.erb. This is because I couldn't get a helper to work.

[pablobm]

@nickcharlton - I made the last few required changes. I think this is good to go. I have two questions though:

• Do you think I'm right in removing these files? 781de9d
• Should I be including the lockfiles in the gemfiles/ directory (as I'm indeed doing here)? I don't know what the state of the art for these things is currently 😕

[nickcharlton]

I am, you'll be happy to hear, finally merging your work here!

I think removing them in fine and if it turns out to not be, is not that significant.

We do indeed include the lock files for Appraisals.