Implements Async requests for WinHTTP

[fabioxgn]

This allows a long running request to be cancelled and also to update the UI without using a thread.

I'm making requests to a longpoll server and the request can take up to 60 seconds and if I close the application it waits until the request is finished so it might take up to 60 seconds to close.

Some details about the implementation:

• When the request is cancelled an EAbort exception is raised.
• There is a new OnAsyncRequestProcess event in TRestClient that is called every second (min wait time for WinHTTP)
• When you free the TRestClient, if Async is True and there is a running request it is cancelled before freeing the connection

Here's an exemple of use:

  RestClient.OnAsyncRequestProcess :=
    procedure(var Cancel: Boolean)
    begin
      Cancel := True; // Set cancel to true to abort the request
    end;

// This will raise an EAbort if the request is canceled 
 RestClient.Resource(CONTEXT_PATH + 'async')
            .Accept('text/plain')
            .Async
            .GET();

[fabriciocolombo]

Great pull request, thank you.