panBy without animation?

HI!

It would be nice to be able to do panBy without the animation.

I'm currently doing this:

``` javascript
L.Map.include({
  panByWithoutAnimation: function(offset) {
    this.fire('movestart');
    this._rawPanBy(L.point(offset));
    this.fire('move');
    return this.fire('moveend');
  }
});
```

But I don't like to rely on a "nonofficial" method: _rawPanBy.

Thanks!
