Rather than trying to submit both format=raw and task=export in the export
button, simply use format=raw only and modify the controller to understand
format=raw as export.

The old way of using JS to set task=export was unable to reset the task and
thus broke all further use of the form controls, because then task=export was
submitted, even though no task was supposed to be used.

Fixes #3

VM now uses the return value of the extended plugin trigger. true is understood as: The plugin handled the controller itself, so virtuemart.php does not instantiate the controller and call the corresponding task => We actually want to return false, so VM calls the controller

V0.1: First working version of the EU sales report plugin, including export functionality for AT and DE