implemented TTL feature

src/main/java/de/svenkubiak/jpushover/apis/Message.java

@@ -224,6 +224,17 @@ public class Message implements API {
         return this;
     }
 
+    /**
+     * 
+     * @param ttl Seconds until this message should be automatically removed from the device. Needs to be positive
+     * @see https://pushover.net/api#ttl
+     * @return Message instance
+     */
+    public Message withTTL(int ttl) {
+        body.put(Param.TTL.toString(), Integer.toString(ttl));
+        return this;
+    }
+
     /**
      * Uses a given proxy for the HTTP requests to Pushover
      *

src/main/java/de/svenkubiak/jpushover/enums/Param.java

@@ -24,7 +24,8 @@ public enum Param {
     TOKEN("token"),
     URL("url"),
     URL_TITLE("url_title"),
-    USER("user");
+    USER("user"),
+    TTL("ttl");
 
     private final String value;