add images-in-css.markdown to _zh-cn
translate images-in-css.markdown into Chinese
Fix issue #918

As reported by @nebloglazov, the article should include the CSS
declarations of the classes being toggled.
Merge pull request #919 from jacquerie/issue-918

Fix issue #918
Merge pull request #916 from chenxsan/media

translate images-in-css.markdown into Chinese