Added invisible title to mobile buttons, to improve accessibility.

@maethu @jone The title is invisible but it's there for screen readers and tools like that.
![bildschirmfoto 2014-05-05 um 14 52 37](https://cloud.githubusercontent.com/assets/157533/2877856/35937108-d454-11e3-9d04-151ad04522c5.png)