Add icon binding to marker directive for custom icon images

We add icon binding to the marker object, for example:
```
<marker icon="m.icon">
```

Structure example:
```
marker = [{
   icon: 'url.png',
   latitude: 15,
   longitude: -120
}]
```

Structure example with options:
```
var size = new google.maps.Size(40,40);

marker = [{
   icon: {
      url: 'url.png',
      scaledSize: size
   },
   latitude: 15,
   longitude: -120
}]
```