added FacebookClient.Subscription to verify get and post subscription

Added some helper methods to deal with RealTime subscription apis. (#218)

* added `FacebookClient.SubscriptionVerifyToken`
* added `FacebookClient.VerifyGetSubscription` void method. 
* added `FacebookClient.VerifyPostSubscription` method. (returns object which is a dynamic json)

There additional overloads so you can pass `verify_token` or `app secret` in parameter instead of setting it in the `FacebookClient` property.

Here is a full example on how to use it in asp.net web forms. subscription.ashx.cs
MVC could use `ActionFilters`.

```c#
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Web;
using Facebook;

namespace WebApplication
{
    /// <summary>
    /// Summary description for subscription
    /// </summary>
    /// <remarks>
    /// InMemoryNoticiationStore, ?list=true and ?clear=true is for debugging purpose
    /// </remarks>
    public class subscription : IHttpHandler
    {
        private static readonly IList<string> InMemoryNoticiationStore = new List<string>();

        public void ProcessRequest(HttpContext context)
        {
            var fb = new FacebookClient
                         {
                             AppSecret = "....",
                             SubscriptionVerifyToken = "...",
                         };

            var request = context.Request;
            var response = context.Response;


            if (request.QueryString["list"] == "true")
            {
                response.Write(string.Join(",", InMemoryNoticiationStore));
                return;
            }
            else if (request.QueryString["clear"] == "true")
            {
                InMemoryNoticiationStore.Clear();
                return;
            }

            if (request.HttpMethod.Equals("GET", StringComparison.OrdinalIgnoreCase))
            {
                try
                {
                    // VerifyGetSubscription will throw exception if verification fails.
                    fb.VerifyGetSubscription(
                        request.Params["hub.mode"],
                        request.Params["hub.verify_token"],
                        request.Params["hub.challenge"]);

                    response.Write(request.Params["hub.challenge"]);
                }
                catch (Exception ex)
                {
                    InMemoryNoticiationStore.Add(ex.StackTrace);
                }
            }
            else
            {
                try
                {
                    // VerifyPostSubscription will throw exception if verification fails.
                    dynamic result = fb.VerifyPostSubscription(
                        request.Params["HTTP_X_HUB_SIGNATURE"],
                        new StreamReader(request.InputStream).ReadToEnd());

                    // result is a json object that was sent by Facebook
                    // for now just call ToString() so it returns the json string
                    string jsonString = result.ToString();

                    // Process the result
                    // for this demo we will just add it to the list
                    InMemoryNoticiationStore.Add(jsonString);
                }
                catch (Exception ex)
                {
                    InMemoryNoticiationStore.Add(ex.StackTrace);
                }
            }
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}
```

Currently these are private in FacebookClient. Should me these constants public?

```c#
private const string SubscriptionHttpXHubSigntureKey = "HTTP_X_HUB_SIGNATURE";

private const string SubscriptionHubChallengeKey = "hub.challenge";
private const string SubscriptionHubVerifyTokenKey = "hub.verify_token";
private const string SubscriptionHubModeKey = "hub.mode";
```