Refactor to make CookieJar iterable.

Hello @alexcrichton. I'm trying to make CookieJar to be simply iterable. Also I added `to_vec(&self) -> Vec<Cookie>` function. It is related to hyperium/hyper#155. Can you please take a look and maybe suggest something more suitable. 	
Could this avoid adding this new public function for now? Is this necessary to have?