Do not show GitHub button if no github_user or github_repo specified

Fixes #51.

Author: mitya57

alabaster/about.html

@@ -15,12 +15,14 @@ <h1 class="logo"><a href="{{ pathto(master_doc) }}">{{ project }}</a></h1>
 <p class="blurb">{{ theme_description }}</p>
 {% endif %}
 
+{% if theme_github_user and theme_github_repo %}
 {% if theme_github_button|lower == 'true' %}
 <p>
 <iframe src="https://ghbtns.com/github-btn.html?user={{ theme_github_user }}&repo={{ theme_github_repo }}&type={{ theme_github_type }}&count={{ theme_github_count }}&size=large"
   allowtransparency="true" frameborder="0" scrolling="0" width="200px" height="35px"></iframe>
 </p>
 {% endif %}
+{% endif %}
 
 {% if theme_travis_button|lower != 'false' %}
 {% if theme_travis_button|lower == 'true' %}