use local libraries for local testing

[espinielli]

When testing locally use local libraries instead of going to CDN.
PRO's

• can be work when offline

CON's

• awful/difficult management of libraries and relevant versions and alignment of URLs.
  Maybe we need

  • a placeholder for the list of libs/version from where to build the <script> tags
    something like

   libs: [
      [<name>, <version #>, <CDN URL>, <local URL>],
      [jquery, 2.0.3, https://ajax.googleapis.com/ajax/libs/jquery/2.0.3/jquery.min.js, js/jquery-2.0.3.min.js],
      ...
   ]

  And then build the HTML tamplate with something like:

  {% if site.dev %} 
    {% for lib in site.libs %}
       <script src="<get CDN URL>"></script>
    {% endfor %}
  {% else %}
     <script src="<get local URL>"></script>
  {% endif %}