Region import and stats

This pull requests adds the region data to the database and creates the statistics for it.

You must delete all the data and run rake db:seed to populate the tables and Wdpa::Importer.import in the console to import again the protected areas table.

#Seeds 
Seeds file was changed. Country is the last table to be populated and includes the join column (region_id).

#Stats 
Stats Module was completely refactored. Now we have Stats::Country, Stats::Regional, Stats::Global.

All the tests were adapted and they are fully working (test/unit/stats.rb)
You can create ActiveRecord relations by passing in the model, not just the ID.
For example, you can replace both these lines of code with:

```
attributes["region"] = Region.where(name: attributes["region"]).first
```

As an aside, I would generally avoid `find_by_*` methods in Rails, as they are not usually best practice.