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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### Update 
Added description of Attributes section in 

- [Resource section](https://github.com/apiaryio/api-blueprint/blob/c785fef34482fe07e72cb945fb81cd306f881b72/API%20Blueprint%20Specification.md#def-resource-section)
- [Action section](https://github.com/apiaryio/api-blueprint/blob/c785fef34482fe07e72cb945fb81cd306f881b72/API%20Blueprint%20Specification.md#def-action-section)

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This PR closes #25 
I was thinking about this is as well, but it seems like Data Structures is correct here as it points to simply using MSON. Below, it says, "Data structures defined in this section **may** be used in any Attributes section." Once transitions are added, the sentence below can be used to specify that data structures may be used in transitions as well.