cloneDate for rangeStart / rangeEnd

"start" and "end" should be cloned. Causes unintended change of "rangeStart" (and "rangeEnd"), thereby not fetching events in the first time when clicking "prev"
Use .data for dayClick

Get data using data-date attribute
Merge pull request #1 from Daxx909/patch-2

cloneDate for rangeStart / rangeEnd