Added support for recognising YAML null values

Previously the YamlDotNet.Dynamic library did not support any of the valid YAML null values.

Now the DynamicYaml object should behave like the strongly typed YAML serializer. I.e. like https://github.com/aaubry/YamlDotNet/blob/master/YamlDotNet/Serialization/NodeDeserializers/NullNodeDeserializer.cs.

The one catch: a generic object cast on a dynamic object, does not invoke the DLR. Thus YAML null values are not supported when doing things like this:

For YAML:
```yaml
---
someField: ~
...
```

Action:
`object someValue = dynamicYaml.someField`

Will return a `DynamicYaml` object instead of null. This I believe is an acceptable edgecase since there is no real reason to cast to `object` when you are trying to access values (you'll want a more concrete type). This behaviour is documented by unit tests.