Added support for Marker With Label library

Added support for the `markerWIthLabel` library from Google Maps Utility Library V3.  See this link for a reference. http://google-maps-utility-library-v3.googlecode.com/svn/tags/markerwithlabel/1.1.8/docs/reference.html 

example: http://google-maps-utility-library-v3.googlecode.com/svn/tags/markerwithlabel/1.1.8/examples/basic.html

I added access to `labelContent` and `labelClass` marker properties to set from controller code like so;

    @json = User.all.to_gmaps4rails do |user, marker|
      marker.title("My title")
      marker.labelContent("My label content")
      marker.json({labelClass: "user_marker_labels"})
    end


  In your view, turn on by adding ```:marker_with_label``` to your marker options then you can add other markerWithLabel properties using the raw option like so;

    gmaps(map_options: {...}, markers: {data: @json, options: { marker_with_label: true, ...},
            raw: '{labelStyle: {opacity: 0.75}, labelAnchor: new google.maps.Point(22, 0)}'}})