Add Chef::Org model class

Chef Server 12 added multi-tenancy to the open source Chef Server. An
organization is the Chef Servers name for a tenant.  The Chef::Org
class provides a model of an organization to facilitate writting knife
commands that can interact with (create, destroy, list) Chef
organizations.

This implementation is copied directly from knife-opc, the knife
plugin currently in use by chef-server-ctl:

  https://github.com/opscode/knife-opc
As we've moved into RSpec 3 territory, we're trying to keep specs up-to-date and consistent with Better Specs guidelines, specifically [using `:let` statements](http://betterspecs.org/#let). You can update this to be

```ruby
let(:org) { Chef::Org.new("an_org") }

describe "initialize" do
  it "is a Chef::Org" do
    expect(org).to be_a(Chef::Org)
  end
end
```

Additionally, it is also recommended to not use "should" in your `it` descriptors because `:should` is deprecated. But I get caught up on how creepy statements without "should" in them are.