Attributes description

**For review ONLY – do not merge**

This Pull Request proposes the way to describe **Resource**, **Action** and arbitrary **message-body** attributes in a reusable way. It also proposes the introduction of "data dictionary" to API Blueprint. In addition changes in this PR suggest that, if a message-body data structure is fully described using the attributes description, an explicit example asset (e.g. a JSON blob) is not needed and can be omitted from a blueprint. 

Please read (and review) following examples:

- [Attributes](https://github.com/apiaryio/api-blueprint/blob/zdne/attributes-description/examples/08.%20Attributes.md)
- [Advanced Attributes](https://github.com/apiaryio/api-blueprint/blob/zdne/attributes-description/examples/09.%20Advanced%20Attributes.md)
- [Data Structures](https://github.com/apiaryio/api-blueprint/blob/zdne/attributes-description/examples/10.%20Data%20Structures.md)

Also, in particular, read (and review) following sections of API Blueprint Specification

- [Attributes section](https://github.com/apiaryio/api-blueprint/blob/zdne/attributes-description/API%20Blueprint%20Specification.md#def-attributes-section)
- [Data Structures section](https://github.com/apiaryio/api-blueprint/blob/zdne/attributes-description/API%20Blueprint%20Specification.md#def-data-structures)

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This PR closes #25 
As a follow-up to [this](https://github.com/apiaryio/api-blueprint/pull/135/files#r20399148), we should probably show an example where an attribute is optional.