Added support for (non-global) CookieContianer 

Added support for a CookieContainer. A number of REST api's use OAuth or other form of cookie authentication so cookie support is essential. Also, the CookieContainer has to be associated with the Url instance and not the global configuration. This is because the client app may have multiple clients accessing the remote resources and you cannot have one global cookie container.

Kept things simple:

var url = new Url(urlString).WithCookieContainer(cookieContainer)....