gets the start of the week
| date | Date | a date object |
| Date | a date object adjusted to the start of the week |
var date1 = new Date(2012,11,12,8,35);
var date2 = webix.Date.weekStart(date1);
console.log(date1); // -> Wed Dec 12 2012 08:35:00 GMT+0300
console.log(date2); // -> Sun Dec 09 2012 00:00:00 GMT+0300