Device token api endpoint (#1)

* Added /device/token handler with associated business logic and storage tests.

* Use crypto rand for user code

Signed-off-by: justin-slowik <justin.slowik@thermofisher.com>

storage/memory/memory.go

@@ -503,3 +503,14 @@ func (s *memStorage) CreateDeviceToken(t storage.DeviceToken) (err error) {
 	})
 	return
 }
+
+func (s *memStorage) GetDeviceToken(deviceCode string) (t storage.DeviceToken, err error) {
+	s.tx(func() {
+		var ok bool
+		if t, ok = s.deviceTokens[deviceCode]; !ok {
+			err = storage.ErrNotFound
+			return
+		}
+	})
+	return
+}