Add UsersGroup observer

Trigger notification service when new user-group relation was created

app/observers/users_group_observer.rb

+class UsersGroupObserver < BaseObserver
+  def after_create(membership)
+    notification.new_group_member(membership)
+  end
+
+  def after_update(membership)
+    notification.update_group_member(membership)
+  end
+end

spec/observers/users_group_observer_spec.rb

+require 'spec_helper'
+
+describe UsersGroupObserver do
+  before(:each) { enable_observers }
+  after(:each) { disable_observers }
+
+  subject { UsersGroupObserver.instance }
+  before { subject.stub(notification: mock('NotificationService').as_null_object) }
+
+  describe "#after_create" do
+    it "should send email to user" do
+      subject.should_receive(:notification)
+      create(:users_group)
+    end
+  end
+
+  describe "#after_update" do
+    before do
+      @membership = create :users_group
+    end
+
+    it "should send email to user" do
+      subject.should_receive(:notification)
+      @membership.update_attribute(:group_access, UsersGroup::MASTER)
+    end
+  end
+end