Add alt tags to images for a11y (#529)

- Add imageAlt to GridBlock
- Set alt equal to `user.caption` for mapped users
- Set alt equal to `post.author` for blog posts
- Update documentation

lib/core/BlogPost.js

@@ -51,6 +51,7 @@ class BlogPost extends React.Component {
                 post.authorFBID +
                 '/picture/?height=200&width=200'
               }
+              alt={post.author}
             />
           </a>
         </div>