Support dragend event for SebmGoogleMapMarker

[sebholstein]

In addition to #70, it should be possible to get notified when the postion (lat/lng) of the marker has changed.

The API should look like this:

<sebm-google-map-marker 
  [latitude]="lat" [longitude]="lng" [markerDraggable]="draggable"
  (dragEnd)="dragEnded($event)"> <!-- $event = {coords: LatLngLiteral} -->
</sebm-google-map-marker>

[gpothier]

Hi, please note that for some reason neither the dragEnd output nor any other output appear in the index of the documentation for the AgmMarker directive (see screenshot). They do appear in the body if one scroll down, though. The same is also true for other directives.