MAVLink app: Send messages on average at a more correct rate and send on first call

This patch fixes two issues:
  * It sends the message on the first call, making sure that the first update gets sent out.
  * It improves the rate scheduling. In an experiment with 0.5, 50 and 250 Hz all rates were correct within 0.3% of the intended rate.

src/modules/mavlink/mavlink_stream.h

@@ -1,6 +1,6 @@
 /****************************************************************************
  *
- *   Copyright (c) 2014 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2014-2016 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -98,7 +98,7 @@ public:
 
 protected:
 	Mavlink     *_mavlink;
-	unsigned int _interval;
+	unsigned int _interval;		//<<< if set to zero = unlimited rate
 
 #ifndef __PX4_QURT
 	virtual void send(const hrt_abstime t) = 0;