Avoid using full-size images for featured images

This patch makes use of the thumbnail version of the featured image, instead of the full-size.
It saves a lot of bandwidth, speeds up the main page loading, and avoids a browser crash on my smartphone (probably because of too much memory usage).
This patch was inspired by http://wordpress.org/support/topic/getting-post-thumbnail-url
Disable Google Web fonts and XHTML Friends Network, to avoid external links in web pages.
This should be enhanced with a theme option, instead of hard-coded
Add initial version of French translation (not 100% complete)
Merge pull request #1 from mossroy/master

Avoid using full-size images for featured images