fix datepicker css for use with changeYear/changeMonth (close #2927)

refs/close #2927

Before:
![before](https://cloud.githubusercontent.com/assets/165599/4808881/f302eb16-5ea5-11e4-90e1-5316c5b696fc.jpg)

After:
![after](https://cloud.githubusercontent.com/assets/165599/4808866/cf8486cc-5ea5-11e4-96bc-fe89a2fdda08.jpg)