Fix 201301mb

Target commit: aace9cd

Description: 
We can not broadcast tweets (or other social messages) due to your checking for the max length of the content written in multibyte languages like Japanese even though it is with the limitation.
Twitter allows multibyte languages to tweet under the counts of the characters, not the size of character bytes.

![WP_plugin_Social_detect_content_length_wrong](https://f.cloud.github.com/assets/1251352/43061/35f4899c-566b-11e2-83c3-8b2545470903.png)

Changes:
strlen -> mb_strlen

***
This is my very first pull request; I may have edited, committed , and pushed in the wrong manner for the  first coulpe of commits.  Only the last commit (aace9cd) should be right on the track.  If this request contains other commits, please discard them.

Those which I've pushed wrong were made as I generated a Japanese language mo file. I would try again with brand-new branch later.