add github icon to navigation

Author: Lisa P

css/main.css

@@ -65,7 +65,7 @@ h1, .h1 {
   background-image: url(../img/icon_twitter.png);
   background-repeat: no-repeat;
   background-position: center center;
-  background-size: 16px 16px;
+  background-size: 16px 14px;
 }
 .icon-ribbon {
   background-image: url(../img/icon_ribbon.png);
@@ -79,6 +79,12 @@ h1, .h1 {
   background-position: center center;
   background-size: 12px 16px;
 }
+.icon-github {
+  background-image: url(../img/icon_github.png);
+  background-repeat: no-repeat;
+  background-position: center center;
+  background-size: 22px 22px;
+}
 .section {
   padding: 30px 0;
 }

img/icon_github.png

@@ -39,11 +39,17 @@
                   </a>
                 </li>
                 <li>
-                  <a href="https://twitter.com/share?url=http://ruby-white-ribbon.org/&lang=" class="external" target="_blank">
+                  <a href="https://twitter.com/share?url=http://ruby-white-ribbon.org/&lang=en" class="external" target="_blank">
                     <i class="icon icon-twitter"></i>
                     <span>Tweet support</span>
                   </a>
                 </li>
+                <li>
+                  <a href="https://github.com/ruby-white-ribbon/ruby-white-ribbon.github.io" class="external" target="_blank">
+                    <i class="icon icon-github"></i>
+                    <span>Github</span>
+                  </a>
+                </li>
               </ul>
             </div>
           </header>