More seamless background images for larger displays

- Made the 4 main hero images subtly fade to black
- Make body background colour black to hide background edges
- Slightly adjusted related background styles to avoid !important
declarations where possible
Remove unnecessary empty body rule
Actually adding updated background images

Ooops.
Merge remote-tracking branch 'upstream/master' into background-images
Add Building web apps with Yeoman and Polymer, profile
Adding link, generator-webapp mention
Removing outdated information

The getResponseHeaders() bug has been fixed in all browsers.
added text compression article.
updated tutorials.yaml
Fixing SVG embed
Merge pull request #641 from jakearchibald/master

Fixing SVG embed
Addressing feedback from Arthur
Merge pull request #627 from addyosmani/master

Fixes #571 - Adds 'Building webapps with Yeoman and Polymer'
Fixes some issues with the article reporting tool
Quick fix to tutorials
Merge pull request #640 from mainroach/master

Fixes #585 - Added "Text compression for web developers" tutorial
Correcting yaml file errors
Merge pull request #611 from WillsB3/background-images

Background images
Merge pull request #638 from monsur/patch-1

Removing outdated information
aligned all google accounts to G+ id