add signature setting

For multiple users it helps to
keep track about who did what.

app/src/main/java/it/danieleverducci/lunatracker/entities/LunaEvent.kt

@@ -55,6 +55,12 @@ class LunaEvent: Comparable<LunaEvent> {
         set(value) {
             jo.put("notes", value)
         }
+    var signature: String
+        get(): String = jo.optString("signature")
+        set(value) {
+            if (value.isNotEmpty())
+                jo.put("signature", value)
+        }
 
     constructor(jo: JSONObject) {
         this.jo = jo
@@ -63,16 +69,18 @@ class LunaEvent: Comparable<LunaEvent> {
             throw IllegalArgumentException("JSONObject is not a LunaEvent")
     }
 
-    constructor(type: String) {
+    constructor(type: String, signature: String) {
         this.jo = JSONObject()
         this.time = System.currentTimeMillis() / 1000
+        this.signature = signature
         this.type = type
     }
 
-    constructor(type: String, quantity: Int) {
+    constructor(type: String, signature: String, quantity: Int) {
         this.jo = JSONObject()
         this.time = System.currentTimeMillis() / 1000
         this.type = type
+        this.signature = signature
         this.quantity = quantity
     }