added $attributes to sync() like attach()

sync is basically a shortcut to a foreach loop using attach.  Now you can pass attributes you want synced as well since it's just using attach() anyways.

I use this in my own application clouddueling all the time to manage a business has employees relationship and give me access when I want to do special things like tracking activity.  (e.g. "This person added this to this.  Would you like to fire them?")