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).


Thanks for clearing that up. I think that documenting it is a good idea, either inline with a note on deprecation or maybe by adding a section near the end of the document describing things like this done for backward compatibility. Right now as a consumer of the AST I have an element returned to me that is not in the spec, which is a bit strange.

I also think that communicating its eventual removal is going to be fairly important :wink: