title: Leaflet-Realtime plugin with GeoJson and multiple markers
body: <p>Hello guys i am trying to update my markers position but i don't manage to find out how to remove the old ones. All i get is a "history" of the marker. I didnt any examples that will help me. I hope someone will give me a clue to go on.
  Many thanks to <a href="http://www.liedman.net/" rel="nofollow">Per Liedman</a> for the awesome job.</p>
  
  <pre><code>                    var shipLayer = L.layerGroup();
                      var ships = L.icon({
                          iconUrl: '../icons/ship-icon.png',
                          iconSize: [30, 30]
                      });
                      var realtime = L.realtime({
                          url: 'jsonServlet/ships.json',
                          crossOrigin: true,
                          type: 'json'
                      }, {
                          interval: 5 * 1000,
                          pointToLayer: function (feature, latlng) {
  
                              marker = L.marker(latlng, {icon: ships});
                 marker.bindPopup('mmsi: ' + feature.properties.mmsi +
                                 '&lt;br/&gt; course:' + feature.properties.hdg+
                                 '&lt;br/&gt; speed:' + feature.properties.sog);
                                 marker.addTo(shipLayer);
                                 return marker;
                          }                            
                      });  
                    controlLayers.addOverlay(geojson, 'Ships');
  </code></pre>