handle Nullable Types

The parser is not compatible with nullable types (int? for example)

It can be done by adding:
```
            if (Nullable.GetUnderlyingType(type) != null)
            {
                return ParseValue(Nullable.GetUnderlyingType(type), json);
            }
```
to
https://github.com/zanders3/json/blob/master/src/JSONParser.cs#L251-L252