Namespace for event binded on document and cleanup activity

Hi Adam,

I have added namespace to events (dragstart, dragstop and mousedown) which are being attached to document, so that they can be un-binded when fullcalendar gets destroyed. 

To attach namespace, I have used <WidgetName><TimeStamp> and hence stored a config option "_createTS" in options object. This is to make sure that I do not un-bind for other fullcalendar ( consider the case where a single page can have multiple full calendar running). Using the namespace, un-binding is being done in destroy function. 

Please review the changes and merge it to main branch if you think changes are good for fullcalendar plugin.

Thanks,
Kunal