add icon customization.  set color or custom asset

This adds the .icon method to the pins.   

currently you can adjust the color setting of the default icon

    var pins = [
            {
                lat: 49.28115,
                lon: -123.10450,
                title: "A Cool Title",
                snippet: "A Really Cool Snippet",
                icon: mapKit.iconColors.HUE_ROSE
            }]

mapkit.iconColors === the static values from BitmapDescriptorFactory


or you can use the fromAsset method

    var pins = [
            {
                lat: 49.28115,
                lon: -123.10450,
                title: "A Cool Title",
                snippet: "A Really Cool Snippet",
                icon: {
                    type: "asset",
                    resource: "www/some_image_in_the_asset_directory.png"
                }
            },