Merge branch '2016-06' of github.com:swcarpentry/styles into 2016-06

_includes/navbar.html

 <nav class="navbar navbar-default">
   <div class="container-fluid">
     <div class="navbar-header">
+      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
+        <span class="sr-only">Toggle navigation</span>
+        <span class="icon-bar"></span>
+        <span class="icon-bar"></span>
+        <span class="icon-bar"></span>
+      </button>
       {% if page.carpentry == "swc" %}
       <a href="{{ site.swc_site }}" class="pull-left">
         <img class="navbar-logo" src="{{ site.github.url }}/assets/img/swc-icon-blue.svg" alt="Software Carpentry logo" />