Apply 'active' class to daterangepicker button when it's open

Consistant with Bootstrap Dropdowns, add bootstrap's `active` class to the button when it's open

![screen shot 2014-03-05 at 3 51 01 pm](https://f.cloud.github.com/assets/97612/2340296/12de5ab4-a4c1-11e3-9567-8ba967492d6e.png)