Attributes description

### For review ONLY – do not merge

---

This PR is implementation of https://github.com/apiaryio/api-blueprint/pull/135 in the API Blueprint AST

### **TODO**

- [x] Extend AST to hold resolved assets and MSON subtrees
- [x] Better examples
- [ ] Source Maps

---

### Update

#### Resolved Properties
In addition to introducing Attributes sections into API Blueprint, this PR also proposes the extensions to API Blueprint AST to support API Blueprint parser / subsequent tools in the form of "resolved" object properties. 

Resolved properties (starting with the "resolved" prefix) are not generated by the parser itself as they do not depict the source blueprint, instead they MAY be generated by the parser harness (or other tools further downstream) to simplify some frequent operations with the API Blueprint AST. Currently, this covers

- Fully resolving (expanding) MSON subtrees in attributes description nodes
- Generating representations or schemas out of attributes description. 

#### YAML Examples
To keep the file manageable I have decided to remove the YAML example. I am considering removing YAML examples from other serializations documents as well (but still want to keep YAML as possible serialization format).


Is this a backward compatible change? It seems like the AST is changing from:

```json
{
  "body": "..."
}
```

to:

```json
{
  "assets": {
    "body": "..."
  }
}
```