Use bootstrap 3.1.1 with responsive layout

I included jquery + boostrap hosted on CDNs - bootstrap javascript requires jQuery and bootstrap javascript was required to get the responsive navbar. When the screen size is small, the navbar shows a pull-down menu instead of showing the top-level navigation links.

If you think this is a little too _heavy_ for such a small feature, we can easily remove them.

The main changes are:
 * meta viewport width in the default page layout
 * navbar uses new bootstrap 3 structure and classes
 * post & page layouts use the new responsive bootstrap3 grid classes