Add Description property to ModelSpec and add IModelFilter.

I believe it would be useful to add support for documentation on models and properties. This is handled by the "description" property on the model spec. Swagger UI currently seems to only display the descriptions for properties, but the description can be provided for models as well.

To set the description property on the models and properties, I created the IModelFilter interface.  This allows complex models to be processed after they are created.

I have included an example in the TestApp; documentation is available for GET Customer. The documentation is provided through ApplyCustomModelDescriptions which implements IModelFilter.