MSON AST changes

- Adopted MSON AST changes from apiaryio/mson-ast#9
- Fixed implicit array bug. Now the following property member will have implicit array type assigned

```
time: 1, 2, 3, 4
```
I understand both elements and one of are backed by the same data structures. But this might be really harder to chew up for a newcomer reading through these comments. Can you please clarify it in it?