adds Budget::Group model to group headings

many refactors through budget related models

app/models/budget/group.rb

@@ -0,0 +1,10 @@
+class Budget
+  class Group < ActiveRecord::Base
+    belongs_to :budget
+
+    has_many :headings, dependent: :destroy
+
+    validates :budget_id, presence: true
+    validates :name, presence: true
+  end
+end