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

spec/models/device_spec.rb

@@ -0,0 +1,15 @@
+require 'rails_helper'
+
+RSpec.describe Device, type: :model do
+  let(:device) { create(:device) }
+
+  it { should validate_presence_of :unique_id }
+  it { should validate_presence_of :model }
+  it { should validate_uniqueness_of :unique_id }
+
+  describe "after create" do
+    it "generates an access token for the device" do
+      expect(device.access_token).not_to be_nil
+    end
+  end
+end