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

> The name can be taken from the resource name later.

It can. And it always will (be needed). Do not see why not to walk towards this. See my comment above.