Add Thumbnails, Emphasis classes, Responsive utilities, Modals, Popovers, Fix require php_markdown

This line was reverted in #27
Also added Thumbnails to close #6
Also added Emphasis classes to close #15 
Also added Responsive Utilities to close #21 
Also added Modals to close #7
Adjusted new data- attributes for buttons, allowing support for popovers without additional shortcode. Closes #8