Add reset to store, optional original instance

1) `get` optionally returns original instance:
``` javascript

storeMethod: function () {
  var object  = {foo: 'bar'};
  this.update('data', object);
  object === this.get('data'); //false
  object === this.get('data', true); //true
};

```

2) `reset` - set store to `getInitial`s result or empty object.