Add cuke for exist matcher.

features/matchers/exist.feature

@@ -0,0 +1,44 @@
+Feature: exist matcher
+
+  The exist matcher is used to specify that something exists
+  (as indicated by #exist?):
+
+    obj.should exist # passes if obj.exist?
+
+  Scenario: basic usage
+    Given a file named "exist_matcher_spec.rb" with:
+      """
+      class Planet
+        attr_reader :name
+
+        def initialize(name)
+          @name = name
+        end
+
+        def inspect
+          "<Planet: #{name}>"
+        end
+
+        def exist?
+          %w[Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune].include?(name)
+        end
+      end
+
+      describe "Earth" do
+        subject { Planet.new("Earth") }
+        it { should exist }
+        it { should_not exist } # deliberate failure
+      end
+
+      describe "Tatooine" do
+        subject { Planet.new("Tatooine") }
+        it { should_not exist }
+        it { should exist } # deliberate failure
+      end
+      """
+    When I run "rspec exist_matcher_spec.rb"
+    Then the output should contain all of these:
+      | 4 examples, 2 failures                |
+      | expected <Planet: Earth> not to exist |
+      | expected <Planet: Tatooine> to exist  |
+