add a getFirstVisibleHour method

Author: Jens Claes

library/src/main/java/com/alamkanak/weekview/WeekView.java

@@ -1390,6 +1390,14 @@ else if (hour * mHourHeight > mHourHeight * 24 - getHeight() + mHeaderTextHeight
         invalidate();
     }
 
+    /**
+     * Get the first hour that is visible on the screen.
+     * @return The first hour that is visible.
+     */
+    public double getFirstVisibleHour(){
+        return -mCurrentOrigin.y / mHourHeight;
+    }
+
 
 
     /////////////////////////////////////////////////////////////////