Introduced the AuthorityType enum class

graphenej/src/main/java/de/bitsharesmunich/graphenej/AuthorityType.java

@@ -0,0 +1,13 @@
+package de.bitsharesmunich.graphenej;
+
+/**
+ * Enum-type used to specify the different roles of an authority.
+ *
+ * @see <a href="https://bitshares.org/doxygen/authority_8hpp_source.html">Authority</a>
+ */
+
+public enum AuthorityType {
+    OWNER,
+    ACTIVE,
+    MEMO
+}