Adding Japanese language files to ver. 2.9.2

Target commit: ab01e7d
Description: Here are the Japanese language files(.po/.mo) for current stable version. It would be nice if you'd include them in the next release.  Thank you!
(Blow is the plugin setting page screen shot in Japanese.)
![social-plugin-in-ja](https://f.cloud.github.com/assets/1251352/1426973/27110e4e-407d-11e3-8bed-178b2b7cd5bb.jpg)