Siox_Segmentation is an external project now

As of 9e5572d(Add a .gitignore file), Siox_Segmentation lives in

	https://github.com/fiji/Siox_Segmentation

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore if Siox_Segmentation is cloned into external/

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Adjust .gitignore

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
externalize-project: optionally create Jenkins job, too

The appropriate SciJava-scripts need to be linked or copied to bin/
to activate that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>