Added userState parameters and properties for asynchronous calls

Was busy porting some of my code over to use RestSharp instead of the standard .NET WebClient object (it was giving me hassles with timeouts etc.) however I noticed that none of the asynchronous methods provided in RestSharp provide any userState/context objects that I could use for storing the state of certain calls. (The library is an XmlRPC one I threw together which needs to return the method name after receiving a response.

Long story short, I figured I would add the missing functionality to allow me to use my library with RestSharp. If you would like to grab the extensions, then here they are, otherwise thanks for a great library.

Regards,
SPARTAN563