Convert ACL fixture to a factory, and add some tests

The fixture was unused, so I took the opportunity to put in a couple
of basic model tests.

test/factories/acls.rb

@@ -0,0 +1,5 @@
+FactoryGirl.define do
+  factory :acl do
+    sequence(:k) { |n| "Key #{n}" }
+  end
+end