MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
[In MSON Type Name](https://github.com/apiaryio/mson/blob/master/MSON%20Specification.md#352-type-name) can refers to both a named type or a based type. Technically a based type can be looked at as a named type except is the part of a standard library. 

Given 

```
# Object (object)
```

```
- propety (Object)
```

and

```
- property (object)
```

are interchangeable. 

Where do you see a problem?