Added MarkerClusterer event binding.

I've added a way to bind the MarkerClusterer's events via a cluster-events attribute on the Markers directive. I hacked this feature together pretty quickly, I'm sure there is a much better way to do this. Here is a quick usage example:

controller/mapCtrl.js
```js
define([
], function mapCtrlModule() {
  'use strict';

  return [
    '$scope',
    function mapCtrl($scope) {
      $scope.map = {
        center: {
          latitude: 43.653218,
          longitude: -79.383173
        },
        
        zoom: 12,

        markers: [{
          latitude: 43.653218,
          longitude: -79.383173
        }, {
          latitude: 43.654118,
          longitude: -79.384373
        }, {
          latitude: 43.654718,
          longitude: -79.378234
        }, {
          latitude: 43.654218,
          longitude: -79.382234
        }, {
          latitude: 43.657218,
          longitude: -79.382234
        }, {
          latitude: 43.657218,
          longitude: -79.389234
        }],
        
        cluster: true,
        
        clusterOptions: {
          gridSize: 60,
          zoomOnClick: false,
          ignoreHidden: true,
          minimumClusterSize: 2
        },

        clusterEvents: {
          'click': function onClusterClick(cluster) {
            console.debug('onClusterClick( cluster )', cluster);
          },

          'mouseout': function onClusterMouseOut(cluster) {
            console.debug('onClusterMouseOut( cluster )', cluster);
          },

          'mouseover': function onClusterMouseOver(cluster) {
            console.debug('onClusterMouseOver( cluster )', cluster);
          }
        }
      };
    }]
});
```

partial/_map.html
```html
<script type="text/ng-template" id="partial/_map.html">
  <google-map center="map.center" zoom="map.zoom" draggable="true">
    <markers models="map.markers" coords="'self'" doCluster="map.cluster" clusterOptions="map.clusterOptions" cluster-events="map.clusterEvents">
    </markers>
  </google-map>
</script>
```