POM: update to ImageJ 2.0.0-rc-2
SaveToOMERO: remove unused import
SaveToOMERO: make menu command unique

ImageJ1 cares about this; otherwise, the menus do unexpected things (one
command will actually call another), and the Command Finder will not
find all the commands with the same name, either.
DefaultOMEROService: remove unused import
DefaultOMEROService: add support for ImagePlus