Add optional email address to author bio and feed.xml

- Fixes #35
- To add additional social network profiles edit _author-bio.html
- Icons come from the Font Awesome webfont. See http://fontawesome.io/icons/ for a complete list

feed.xml

@@ -13,7 +13,7 @@ sitemap: false
 <author>
   <name>{{ site.owner.name }}</name>
   <uri>{{ site.url }}/</uri>
-  <email>{{ site.owner.email }}</email>
+  {% if site.owner.email %}<email>{{ site.owner.email }}</email>{% endif %}
 </author>
 {% for post in site.posts limit:20 %}