Click events on week numbers and table headers (except week number header)

I added click events to week numbers and table headers to allow easier navigation through the calendar. The callbacks can be set using the `weekClick` and `headerClick` options. Of course the events can be used for any purpose besides navigation, there are no default callbacks included. This fixes parts of Issue 153, the navigation dropdowns are still missing. Maybe I will add that later if we require it for our project.

#### weekClick
Arguments:
**week**: Week container (.fc-week) of the clicked week number
**ev**: Native JS event object
**view**: FullCalendar View object

#### headerClick
Arguments:
**ev**: Native JS event object
**view**: FullCalendar View object