MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
@fosrias 

> Is this correct? Shouldn't this just be a type_name?

It is correct. Type Definition it is. 

> In what case is it ever a Type Definition? It is not in the spec.

It is  : https://github.com/apiaryio/mson/blob/master/MSON%20Specification.md#51-mixin-type

> Mixin Type → - Include Type Name | - Include Type Definition

E.g. 

```
- Include (Address, fixed)
```

And since Type Definition covers Type Name we need just that