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:**

- [ ] Better examples
- [ ] Source Maps

So I was tempted to defined attributes like `Attributes (Named Type)` overloading the properties and descriptions there. To me an Attribute in API Blueprint is identical data structure to what a Named type is, except sometimes it is anonymous. 

When defined under a named resource this `name` could be set to ease the manipulation of the subsequent tools – it does not obfuscates what was written in blueprint in any way.