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.get("blogs.blog.0.needspassword") // true wrapped in JSValue
json.set("blogs.blog.0.needspassword", obj: false)
json.get("blogs.blog.0.needspassword") // false wrapped in JSValue
```

Let me know what do you think about this.