add enum for tone

7 tones used according to https://www.ibm.com/blogs/watson/2017/07/ibm-expands-watson-tone-sensitivity-for-customer-service/

src/main/java/de/fuberlin/imp/memorybox/restmembox/model/Tone.java

+package de.fuberlin.imp.memorybox.restmembox.model;
+
+/**
+ * Possible tones returned from the Watson API.
+ */
+public enum Tone {
+    FRUSTRATION, IMPOLITENESS, SADNESS, SYMPATHY, POLITENESS,
+    SATISFACTION, EXCITEMENT
+}