fix(dateRange): Use $scope.fromDate and $scope.toDate instead of dateRange

This removes dateRange object from gantt factory, in favor of more direct use of
$scope.fromDate and $scope.toDate. task-out-of-range default value has been changed to truncate.

Setting only from-date or to-date will works as expected. When no date is defined, gantt internal range is set for that all tasks to be visible. This range is available through $scope.gantt.from and $scope.gantt.to properties.

This also adds smalls optimizations and fix an issue related to auto-expand option.

Closes #169
fix(demo): Add missing boostrap icons and make datepicker pop at current date

Bootstrap icons have been added.

When opening datepickers for date range configuration, it now opens at current date from the demo.