Consider moving main method in test scope (reduce dependency footprint)

[imagejan]

Having a main method in the example command that calls final ImageJ ij = new ImageJ(); requires a dependency on net.imagej:imagej.

In order to help projects inheriting from this example to keep their dependency footprint minimal, we should maybe move that main method into a test class and make the imagej dependency test scope only.

What do others think of that idea? @ctrueden @etadobson @frauzufall ?