Allow to pass object to fromJSON

Use case scenario:

Server renders:

``` html

window.__PRELOADED_STATE__ = ${transit.toJSON(preloadedState)};

```

Client picks up `__PRELOADED_STATE__`, which is a valid JSON, therefore it is converted to an object. To use it, I need to cast it back to string, e.g.

``` js
console.log('window.__PRELOADED_STATE__', transit.fromJSON(JSON.stringify(window.__PRELOADED_STATE__)));
```

Alternatively, I can stringify the object twice on server: once using `transit-immutable-js` and
second time using `JSON.stringify`, e.g.

``` html

window.__PRELOADED_STATE__ = ${JSON.stringify(transit.toJSON(preloadedState))};

```

Preferably though,`transit-immutable-js` should be able to accept an object.