Add GitHub link to site header (#77)

Author: jshbrntt

src/app/shared/navbar/navbar.html

@@ -8,5 +8,10 @@
   </a>
   <a md-button routerLink="components">Components</a>
   <a md-button routerLink="guides">Guides</a>
+  <a md-button href="https://github.com/angular/material2" aria-label="GitHub Repository">
+    <img class="docs-github-logo"
+         src="../../../assets/img/homepage/github-circle-white-transparent.svg"
+         alt="GitHub">
+    GitHub
+  </a>
 </nav>
-

src/app/shared/navbar/navbar.scss

@@ -1,9 +1,23 @@
 .docs-navbar {
+  display: flex;
+  flex-wrap: wrap;
   padding: 8px 16px;
+
+  > .mat-button {
+    &:last-child {
+      margin-left: auto;
+    }
+  }
 }
 
 .docs-angular-logo {
-  margin: 0 4px 3px 0;
   height: 26px;
+  margin: 0 4px 3px 0;
+  vertical-align: middle;
+}
+
+.docs-github-logo {
+  height: 21px;
+  margin: 0 7px 2px 0;
   vertical-align: middle;
 }