add be_able_to matcher

lib/cancan/matchers.rb

@@ -0,0 +1,13 @@
+Spec::Matchers.define :be_able_to do |action, subject|
+  match do |model|
+    model.can?(action, subject)
+  end
+
+  failure_message_for_should do |model|
+    "expected to be able to #{action.inspect} #{subject.inspect}"
+  end
+
+  failure_message_for_should_not do |model|
+    "expected not to be able to #{action.inspect} #{subject.inspect}"
+  end
+end