message: add facebook open graph and twitter social cards to public website
diff: <script src="js/credentials.js"></script>
  <script src="https://unpkg.com/three/build/three.min.js"></script>
  <script src="https://unpkg.com/@here/harp.gl/dist/harp.js"></script>
  +
  + <meta name="twitter:card" content="summary" />
  + <meta name="twitter:site" content="@here" />
  + <meta name="twitter:title" content="harp.gl" />
  + <meta name="twitter:description" content="3d web map rendering engine" />
  + <meta name="twitter:image" content="http://harp.gl/resources/background.png" />
  +
  + <meta property="og:url" content="http://harp.gl" />
  + <meta property="og:type" content="website" />
  + <meta property="og:title" content="harp.gl" />
  + <meta property="og:description" content="3d web map rendering engine" />
  + <meta property="og:image" content="http://harp.gl/resources/background.png" />
  </head>
  <body>
  <header>
  <canvas id="map"></canvas>
  -
  <div class="container header-container">
  <nav>
  <div class="logo"></div>
  