Added HourClickListener interface

This solves the problem #13 (How to add a new event by tapping on an empty view ?)

When a user click an hour block that has no event registered, weekview will call onHourClicked(Calendar startTime) in HourClickListener interface.

    public interface HourClickListener{
            public void onHourClicked(Calendar startTime);
    }

In using this functionality, you just need to implement HourClickListener interface, like what you did to MonthChangeListener interface.