hide-text for non-English Characters

This mixin will not work perfectly if line-height is 1 for Japanese. (maybe Chinese also).
Set line-height to 1.X will be safer.

![2014-10-21 14 06 15](https://cloud.githubusercontent.com/assets/2394070/4713397/7845d200-58e0-11e4-9747-b2f7f232a9e3.png)