Make toEqual matcher report the difference between objects

- Mismatches deep within object/array structures are pinpointed with a JsonPath-like syntax.

spec/core/matchers/NullDiffBuilderSpec.js

@@ -0,0 +1,13 @@
+describe('NullDiffBuilder', function() {
+  it('responds to withPath() by calling the passed function', function() {
+    var spy = jasmine.createSpy('callback');
+    jasmineUnderTest.NullDiffBuilder().withPath('does not matter', spy);
+    expect(spy).toHaveBeenCalled();
+  });
+
+  it('responds to record()', function() {
+    expect(function() {
+      jasmineUnderTest.NullDiffBuilder().record('does not matter');
+    }).not.toThrow();
+  })
+});