Add heartbeat model, next_heartbeat_time and update_heartbeat_time on device registration

db/migrate/20151109071857_create_heartbeats.rb

@@ -0,0 +1,9 @@
+class CreateHeartbeats < ActiveRecord::Migration
+  def change
+    create_table :heartbeats do |t|
+      t.references :device, index: true, foreign_key: true
+
+      t.timestamps null: false
+    end
+  end
+end