Add support for getting/setting range in Calendar when the selectable option is set to 'range'.

Scenarios:

1. Setting the start and end date of the range in the Calendar.
   Example:

var that = $("#calendar").data('kendoCalendar')
that.rangeSelectable.range(new Date(25, 0, 5), new Date(25, 0, 20));

2. Having the range method would allow correct retrieving of the range when it is changed. Currently, the range could no be retrieved as the value() method returns the previous selected date - https://dojo.telerik.com/PpmQRlRY