Create device model and setup devise, activeadmin and rspec!

- with attributes: model, imei_number, unique_id and access_token
 - access_token is automatically generated on create

app/models/device.rb

@@ -0,0 +1,9 @@
+class Device < ActiveRecord::Base
+  validates :unique_id, :model, presence: true
+  validates :unique_id, uniqueness: true
+  before_create :generate_access_token
+
+  def generate_access_token
+    self.access_token = SecureRandom.uuid
+  end
+end