add links to footers of oscar and simple themes

Links:
 * Source code
 * Issue tracker
 * List of public instances

Closes #1071

searx/templates/oscar/base.html

         <div class="container">
             {% block footer %}
             {% endblock %}
-            <p class="text-muted"><small>{{ _('Powered by') }} <a href="https://asciimoo.github.io/searx/">searx</a> - {{ searx_version }} - {{ _('a privacy-respecting, hackable metasearch engine') }}</small></p>
+            <p class="text-muted">
+                <small>
+                    {{ _('Powered by') }} <a href="https://asciimoo.github.io/searx/">searx</a> - {{ searx_version }} - {{ _('a privacy-respecting, hackable metasearch engine') }}<br/>
+                    <a href="https://github.com/asciimoo/searx">{{ _('Source code') }}</a> |
+                    <a href="https://github.com/asciimoo/searx/issues">{{ _('Issue tracker') }}</a> |
+                    <a href="https://github.com/asciimoo/searx/wiki/Searx-instances">{{ _('Public instances') }}</a>
+                </small>
+            </p>
         </div>
     </div>
     <script src="{{ url_for('static', filename='js/jquery-1.11.1.min.js') }}"></script>

searx/templates/simple/base.html

     {% endblock %}
   </main>
   <footer>
-    <p>{{ _('Powered by') }} <a href="{{ url_for('about') }}">searx</a> - {{ searx_version }} - {{ _('a privacy-respecting, hackable metasearch engine') }}</p>
+    <p>
+        {{ _('Powered by') }} <a href="https://asciimoo.github.io/searx/">searx</a> - {{ searx_version }} - {{ _('a privacy-respecting, hackable metasearch engine') }}<br/>
+        <a href="https://github.com/asciimoo/searx">{{ _('Source code') }}</a> |
+        <a href="https://github.com/asciimoo/searx/issues">{{ _('Issue tracker') }}</a> |
+        <a href="https://github.com/asciimoo/searx/wiki/Searx-instances">{{ _('Public instances') }}</a>
+    </p>
   </footer>
   <!--[if gte IE 9]>-->
   <script src="{{ url_for('static', filename='js/searx.min.js') }}" ></script>