Addresses issue #11 items unit testing, snapshot testing. Follows instructions from https://www.gatsbyjs.com/docs/how-to/testing/unit-testing/ ; Adding typescript config, mocks, snapshot capabilities; initial test for j40footer and snapshot, readme update, test helper for intl plugin (#79)

client/__mocks__/file-mock.js

@@ -0,0 +1,4 @@
+/* This was created following along from
+https://www.gatsbyjs.com/docs/how-to/testing/unit-testing/
+The purpose is to mock static file imports - see jest.config.js */
+module.exports = 'test-file-stub';

client/__mocks__/gatsby.js

@@ -0,0 +1,27 @@
+const React = require('react');
+const gatsby = jest.requireActual('gatsby');
+
+module.exports = {
+  ...gatsby,
+  graphql: jest.fn(),
+  Link: jest.fn().mockImplementation(
+      // these props are invalid for an `a` tag
+      ({
+        activeClassName,
+        activeStyle,
+        getProps,
+        innerRef,
+        partiallyActive,
+        ref,
+        replace,
+        to,
+        ...rest
+      }) =>
+        React.createElement('a', {
+          ...rest,
+          href: to,
+        }),
+  ),
+  StaticQuery: jest.fn(),
+  useStaticQuery: jest.fn(),
+};