Joystick: update gui and send new mavlink target messages

This PR contains work from @julianoes and me which updates the joystick input implementation.

@julianoes added a nice calibration for the joystick.

When the joystick input  is activated, now the new mavlink target (local position target etc) messages are sent
The mode (attitude, position, force, ...) can be set in the gui

![screenshot from 2014-08-11 14 19 36](https://cloud.githubusercontent.com/assets/1419479/3875433/e1a5415c-2151-11e4-89cd-beaffce9b7f0.png)


The PR contains #811 

The PR will probably conflict #808