Added support for reading multiple objects from single stream

[A question on SO](http://stackoverflow.com/q/18805735/41071) asked how to read multiple JSON objects from single stream. I believe JSON.NET isn't currently able to do this. And I think this is not the first time someone asked for this, so I thought implementing it could be useful.

I have made it so that this behavior is enabled when `CheckAdditionalContent` is `false`, which more or less makes sense to me. But using a new option for this would be another option.

With this change, code like this works:

    var reader = new JsonTextReader(new StringReader(@"{ ""name"": ""bar"" }{ ""name"": ""baz"" }"));
    var serializer = new JsonSerializer { CheckAdditionalContent = false };
    var foo1 = serializer.Deserialize<Foo>(reader);
    var foo2 = serializer.Deserialize<Foo>(reader);