SwiftJSON update feature implemented

Just tried to achieve modification feature especially for https://github.com/SwiftyJSON/SwiftyJSON/issues/60
Maybe api should be refined, but now it works like this:
```
let json = JSON(object: [["user":["name":"Anton"]],2,3])
println("Before: \(json)")

let json2 =
json.updated(0) {
    $0.updated("user") {
        $0.updated("name") {
            JSON(object: $0.stringValue + " Bukov")
        }
    }
}
println("After: \(json2)")
```
Output:
```
Before: [{
    user =     {
        name = Anton;
    };
}, 2, 3]
```
```
After: [{
    user =     {
        name = Anton Bukov;
    };
}, 2, 3]
```