Hide overflowing header texts

To solve the [problem](https://github.com/Serhioromano/bootstrap-calendar/issues/128) reported by @gdscei: let's avoid header texts out of cell bounds:

![narrow-view](https://f.cloud.github.com/assets/928116/1251874/8b7c7500-2b3b-11e3-9889-2ec714931a43.png)