can read GeoJson features (geometry + properties)

This feature adds the capability to parse GeoJSON features. According to [the GeoJSON spec](http://geojson.org/), Features are *Geometries with additional properties*, so the following should be a valid GeoJSON object:

```js
{
  "type": "Feature",
  "geometry": {
    "type": "Point",
    "coordinates": [125.6, 10.1]
  },
  "properties": {
    "name": "Dinagat Islands"
  }
}
``` 

In turn, Wicket will parse this as a point (or whatever geometry type it results by drilling on the object 'geometry' key), then call the framework constructor and finally add a *properties* key.

```js
{
    "gm_accessors_":{
        "position":null,
        "clickable":null,
        "visible":null
    },
    "position":{
        "d":10.1,
        "e":125.6
    },
    "gm_bindings_":{
        "position":{},
        "clickable":{},
        "visible":{}
    },
    "clickable":true,
    "visible":true,
    "properties":{
        "name":"Dinagat Islands"
    }
}
```

This is perfectly safe for Google Maps objects. To my knowledge, there isn't a reserved *properties* key in Leaflet and Argcis objects either.