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
translation of site UI strings into Korean.
Adding link, generator-webapp mention
upload mo file.
Removing outdated information

The getResponseHeaders() bug has been fixed in all browsers.
update UI String
added text compression article.
updated tutorials.yaml
Addressing feedback from Arthur
fix unmatched msgid & msgstr
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