adds rest list receiver, adds local code storage

- rest class now receives a list as well as single
  messages
- adds support for threaded singleton which takes
  care of continuous checking of the bump server
  to get the most up-to-date list of messages
- adds local data class with tests to get locally saved
  code in preferences

app/src/main/res/values/strings.xml

@@ -12,4 +12,6 @@
     <string name="current_sender">No Sender Added</string>
     <string name="scan_sender">Scan</string>
     <string name="camera_fragment_label">Camera Fragment</string>
+    <string name="preference_file_key">code_file_key</string>
+    <string name="code_key">code_key</string>
 </resources>