Renamed class .switch to .datepicker-switch

Renamed class .switch used in Datepicker to .datepicker-switch as Bootstrap already has a class named .switch and it conflicts with Datepicker's rendering.

Screenshot courtesy of @michahell:
![Snap 2013-04-02 at 07 39 07](https://f.cloud.github.com/assets/2670603/328025/04770994-9b8a-11e2-84df-31c9e4f8a2be.png)