Adding the capability to mark properties as required, and to have them be arrays

Model Properties can now be:
* required
* carrying a description
* arrays (both of basic types, and of `$refs` / other Models

See the following screenshot with how the new sections look in the Swagger UI:

![](http://content.screencast.com/users/tim.schmelmer/folders/Jing/media/8bf0488d-5731-451b-b7ed-c14b9f8a0562/00000255.png)

also :+1: here, ... I was just unable to coerce `Yard::Tag` into cooperating, or to find an appropriate tag in the library.