Add optional social sharing buttons

- Social share buttons appear at the end of every post and page unless you add share: false to their YAML Front Matter
- Fixes #42

_layouts/page.html

@@ -36,6 +36,10 @@
     <h1>{{ page.title }}</h1>
     <div class="article-wrap">
       {{ content }}
+      {% if page.share != false %}
+        <hr />
+        {% include _social-share.html %}
+      {% endif %}
     </div><!-- /.article-wrap -->
     {% if site.owner.disqus-shortname and page.comments == true %}
       <section id="disqus_thread"></section><!-- /#disqus_thread -->

_layouts/post.html

@@ -44,6 +44,7 @@
       {{ content }}
       <hr />
       <footer role="contentinfo">
+        {% if page.share != false %}{% include _social-share.html %}{% endif %}
         <p class="byline"><strong>{{ page.title }}</strong> was published on <time datetime="{{ page.date | date_to_xmlschema }}">{{ page.date | date: "%B %d, %Y" }}</time>{% if page.modified %} and last modified on <time datetime="{{ page.modified | date: "%Y-%m-%d" }}">{{ page.modified | date: "%B %d, %Y" }}</time>{% endif %}.</p>
       </footer>
     </div><!-- /.article-wrap -->