YARD handler for Spira properties

I’ve written a rudimentary YARD handler which allows you to document Spira `property` and `has_many` definitions in the same way as you would with `attr_accessor`.

I’m pretty sure I haven’t done it right—the handler isn’t in a module (it’s just at the top level), it’s in a file called `spira/yard.rb` (you might want to change that), and the system currently needs you to `require 'spira/yard'` in your Rakefile or yardopts. I figured it would be best to let you decide where to put it, but at least the functional code is there. In fact, I’ve been using it myself for the past few hours.