Feature/clarify misc scenarios

@zdne @pksunkara @smizell @honzajavorek Please review and merge.

This PR clarifies a number of scenarios that we ambiguous or missing when updating examples in https://github.com/apiaryio/mson/pull/6. 

- Adds quick links
- Cleans up structure of spec to make more sense in context of hierarchical list.
- Clarifies escaping keywords
- Adds wildcard type name
- adds Default functionality and aligns with Sample functionality with 3 flavors each

DO NOT MERGE until https://github.com/apiaryio/mson/pull/6 is merged.

Sorry I wasn't precise by

> Sample is mutually exclusive with Default

 I have meant in the attribute list (not in the sections). This is what I mean:

```
- address: none (string, required, default, sample)
```