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

In the month view, the Kendo-UI Calendar widget displays the days of the next or previous month to fill the remaining space in a week. When a date is selected, it is highlighted.

Imagine the following scenario:

• You select the first day of any month.
• You view the month before the one with the selected day in the month-view. As long asthe last day of this month is not a Sunday, you should also see an entry for the date you selected before.
• However, the currently selected date is not highlighted.

This can easily be repoduced with the demo calendar.