Added a check that getDaySegmentContainer isn't undefined when clearEvents is called.

I use the calendar along with bootstrap tabs and AngularJS and AngularUI. The calendar is placed in a tab that is isn't active from the start and is render when the calendar tab is clicked. I added this check to get rid of the error *TypeError: Cannot call method 'empty' of undefined* of the ```getDaySegmentContainer```. However, this might not be a general problem.

Anyway, thanks for a great calendar.
// John