Added option to animate pan on invalidateSize()

[ajbeaven]

I thought it might be nice to have the option to animate the map movement when invalidateSize is called rather than the map jumping to its new position.

[nicooprat]

It looks like passing :

options: {
    animate: false
}

to invalidateSize() has no effect anymore ; the animation is always triggered. I took a look in leaflet.js and found that _rawPanBy is called and seems to animate anyway (no options concerning animation is passed).

Any idea on this ?