Bring the have(n).items matcher from rspec-expectations

This PR brings the rspec `have(n).items` matchers from rspec-expectations into this gem. This PR is the very beginning of the task of building that gem. The scope of that PR is just to bring back the specs from those matchers  from rspec-expectation and make them green inside this new gem.

After that PR, I still need to do stuff like configuring Travis as a CI for that gem, and setting up the proper warnings for users migrating from RSpec 2.14.0 to RSpec 2.99 and then to RSpec 3.0. I'm planning to add those tasks as github issues of that repo, so that I can track what I need to do.