added:  'onSet' & 'onRemove' callback support

added:  'onSet' & 'onRemove' callback support
Since the code checks that it is present before it calls it, we don't need a dummy function as default. That will only impose extra overhead for all existing uses of `$.cookie` by calling a useless function. Since the default is a no-op, users will only pass a function if they want to do something special (not to disable the option, because it doesn't do anything by default). So either remove the `if` check and keep the default, or remove the default and keep the `if` check. I'd recommend the latter.