Added <JSValue>.get and <JSValue>.set

Added a way to get and set JSON using path

```
var json: JSON = [
            "stat": "ok",
            "blogs": [
                "blog": [
                    [
                        "id": 73,
                        "name": "Bloxus test",
                        "needspassword": true,
                        "url": "http://remote.bloxus.com/"
                    ],
                    [
                        "id": 74,
                        "name": "Manila Test",
                        "needspassword": false,
                        "url": "http://flickrtest1.userland.com/"
                    ]
                ]
            ]
        ]

json.get("blogs.blog.0.url")   // "http://remote.bloxus.com/" wrapped in JSValue
json.set("blogs.blog.0.needspassword", obj: false)
```

Let me know what do you think about this.