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/java/com/example/bump/LocalData.kt

@@ -0,0 +1,24 @@
+package com.example.bump
+
+import android.content.Context
+import android.content.Context.MODE_PRIVATE
+import android.content.SharedPreferences
+import com.maenle.bump.R
+
+class LocalData(context: Context) {
+    private val codeKey = context.getString(R.string.code_key)
+    private val preferenceKey = context.getString(R.string.preference_file_key)
+
+    private val preferences: SharedPreferences =
+        context.getSharedPreferences(preferenceKey, MODE_PRIVATE)
+
+    var code: String?
+    get() {
+        return preferences.getString(codeKey, null)
+    }
+    set(codeValue) {
+        preferences.edit().putString(codeKey, codeValue).apply()
+    }
+
+    // var messages: List<>
+}