[ADD] calendar week option

ability to show/hide week number on the left of the date widget.

Inspired from eternicode bootstrap datepicker: https://github.com/eternicode/bootstrap-datepicker/tree/release

preview: 
![calendarweeks](https://cloud.githubusercontent.com/assets/7338483/3892820/8070124c-2235-11e4-9e33-6cadf6d692dc.png)