Add a main-class to the updater-core
Add a main-class to the updater-core

The command-line updater is a main-class and for convenience, should
be launched when ij-updater-core is called via 'java -jar' or 'mvn
exec:java'.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

TehSAUCE/imagej,TehSAUCE/imagej,biovoxxel/imagej,TehSAUCE/imagej,biovoxxel/imagej,biovoxxel/imagej