Add Sluggable concern and unique validation to Budget, Group and Heading

* What: We need to generate slug on Budget, Group and Heading classes, validating its unique for its scope

* How: Adding a presence and unique validation using Budget always as scope.

app/models/budget/heading.rb

@@ -1,12 +1,15 @@
 class Budget
   class Heading < ActiveRecord::Base
+    include Sluggable
+
     belongs_to :group
 
     has_many :investments
 
     validates :group_id, presence: true
-    validates :name, presence: true
+    validates :name, presence: true, uniqueness: { if: :name_exists_in_budget_headings }
     validates :price, presence: true
+    validates :slug, presence: true, format: /\A[a-z0-9\-_]+\z/
 
     delegate :budget, :budget_id, to: :group, allow_nil: true
 
@@ -16,5 +19,9 @@ class Budget
       "#{group.name}: #{name}"
     end
 
+    def name_exists_in_budget_headings
+      group.budget.headings.where(name: name).any?
+    end
+
   end
 end