Improve semantic structure and content of the HTML

* Fill title and description meta tag
* Change <div> tags by <sections> and used <button> instead of links
  for the buttons
Add favicon and apple-touch-icon
Convert px to em to improve accesibility
Add missing elements from psd

* add two photos
* add box-shadow
Added Media Queries for Responsive design with a stacked cards pattern. Replaced main cards with alpha bg PNGs
Creation of RSVP message element to provide feedback. Added JavaScript handlers for invitation buttons
Updated README.md with latest changes: rsvp interaction
Cached jQuery selectors in variables
Removed unnused JavaScript file: plugins.js
Added gracefully degraded htmls for disabled Javascript
Update README.md
Merge branch 'stacked' into challenge

Merging Responsive behavior from branch 'stacked' into 'challenge'
Fixed z-index on invitation message and added some bottom margin on the buttons
Fixed vertical scroll on mobile screens
removed unused css
updated README.md
minor code formatting changes
removed reference to plugins.js from index.html
Merge pull request #1 from ferlores/challenge

Challenge