added hash to appointment, implemented KassaBlanca parser

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

de.srsoftware.cal.api/src/main/java/de/srsoftware/cal/api/Appointment.java

@@ -34,6 +34,12 @@ public interface Appointment {
 	 */
 	Optional<LocalDateTime> end();
 
+	/**
+	 * create a unique identifier based on the event content
+	 * @return
+	 */
+	String hash();
+
 	/**
 	 * ID of the appointment – unique within this system
 	 * @return the appointment`s id