Allow locale to be set through the front-end

* Store the last used locale in the session
* Allow home page to be identified in helpers even when containing
  parameters
* Create initializer to explicitly set the available locales, the
  default and the load_path

spec/features/localization_spec.rb

@@ -0,0 +1,27 @@
+require 'rails_helper'
+
+feature 'Localization' do
+
+  scenario 'Wrong locale' do
+    visit root_path(locale: :es)
+    visit root_path(locale: :klingon)
+
+    expect(page).to have_text('Estamos abriendo Madrid')
+  end
+
+  scenario 'Changing locale' do
+    visit root_path(locale: :es)
+    locale_switcher = find('#locale-switcher')
+
+    expect(page).to have_text('Estamos abriendo Madrid')
+    expect(locale_switcher).to have_text('en')
+    expect(locale_switcher).to_not have_text('es')
+
+    find('#locale-link-en').click
+    locale_switcher = find('#locale-switcher')
+
+    expect(page).to have_text('We are opening Madrid')
+    expect(locale_switcher).to have_text('es')
+    expect(locale_switcher).to_not have_text('en')
+  end
+end