custom header buttons

Allow for custom header buttons and custom functions declared in the constructor like so:

```
	that.$('#calendar').fullCalendar({
		header: {
			left: 'prev,next today {foo:fooFunction}',
			center: 'title',
			right: 'resourceDay,month,agendaWeek,agendaDay'
		},
		fooFunction: function() {
			console.log('fooFunction');
		}
	});
```

This will add a button to the header (next to "today") with the title "foo" that fires `fooFunction()` on click.