Added drag functionality; update lat/lng on marker "dragend" event.

After the user clicks "Add me to the map" her or she can add a marker to the map.  I've modified the code slightly to allow the user to drag that if it needs to move slightly to a more exact position.  The hidden lat/lng text input fields on the marker popup form are automatically updated with the marker's new position.

I'm relatively new to GitHub; this is my first pull request!