Failing test for User.visits_by_country
visits_by_country returns hash with integers, solves pie chart issue

- visits_by_country uses ActiveRecord methods in user.rb
- pie_chart.js.coffee adapted for hash coming from controller
- user_test.rb updated
all_visits_by_country adapted to ActiveRecord count in city.rb
added test for City::all_visits_by_country
Merge pull request #17 from unepwcmc/user-pie-chart

User pie chart fix