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)
Same as above for the iucn categories. If that's the case, you could get rid of the IUCN_CATEGORIES constant entirely (I would suggest replacing it with a database query anway -- `IucnCategory.pluck(:name)`).