add cookie management

[poef]

It should be easy to add / update / delete cookies
In addition if a request returns a cookie, the client should remember the cookie for following requests to the same host ( depending on the path and other info in the cookie )

[poef]

You can set cookies through the options array, as a stream_context, but userfriendly it isn't. I think it would be best if we had a special cookie controller that you could access per client:

$client->cookies->add(...)
$client->cookies->ls()

But it might be even better to have a faux array for cookies:

$client->cookies[] = \arc\http::cookie( ... );

This might lead to problems because there is a difference between cookies set in the client ( browser ) and cookies send/received for any specific request.

So perhaps we need to wrap the requestHeaders and responseHeaders and let them handle cookies through a simple API as well.