Resizable: Ability to update size and position in resize event

[scottgonzalez]

elem.resizable({
    resize: function( event, ui ) {
        ui.size.width = 100;
    }
});

is a supported API, but isn't documented.

[scottgonzalez]

I have a pending PR for this, just waiting for review on #253 since this basically builds on top of that.