title: Error when using ical4j.jar Java
body: <p>I want to use ical4j in order to send event to users. 
  When I execute this code : </p>
  
  <pre><code>    CompatibilityHints.setHintEnabled(CompatibilityHints.KEY_RELAXED_UNFOLDING, true);
      // Create a TimeZone
      TimeZoneRegistry registry = TimeZoneRegistryFactory.getInstance().createRegistry();
      TimeZone timezone = registry.getTimeZone("Europe/Paris");
      VTimeZone tz = timezone.getVTimeZone();
  </code></pre>
  
  <p>I getting this error ;</p>
  
  <pre><code>Exception in thread "main" java.lang.NoClassDefFoundError: edu/emory/mathcs/backport/java/util/concurrent/ConcurrentHashMap
  at net.fortuna.ical4j.util.CompatibilityHints.&lt;clinit&gt;(CompatibilityHints.java:85)
  at net.fortuna.ical4j.model.Iso8601.&lt;init&gt;(Iso8601.java:78)
  at net.fortuna.ical4j.model.Date.&lt;init&gt;(Date.java:133)
  at net.fortuna.ical4j.model.DateTime.&lt;init&gt;(DateTime.java:244)
  at jconge_batches.TestCalendar.main(TestCalendar.java:106)
      Caused by: java.lang.ClassNotFoundException:       edu.emory.mathcs.backport.java.util.concurrent.ConcurrentHashMap
  at java.net.URLClassLoader$1.run(Unknown Source)
  at java.security.AccessController.doPrivileged(Native Method)
  at java.net.URLClassLoader.findClass(Unknown Source)
  at java.lang.ClassLoader.loadClass(Unknown Source)
  at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
  at java.lang.ClassLoader.loadClass(Unknown Source)
  ... 5 more
  </code></pre>
  
  <p>I have added ical4j.jar in my classpath.</p>
  
  <p>How I can resolve this error?</p>