Reintegrate version management into pom-imagej

This unifies the Bill of Materials (i.e., dependency management) for all
artifacts in the net.imagej, net.imglib2 & io.scif groupIds (i.e., the
imagej, imglib & scifio GitHub orgs, respectively) back into one place.

The pom-imglib2 and pom-scifio parents will now extend pom-imagej to
inherit the complete Bill of Materials.

For the discussion and rationale relating to this change, see:
http://imagej.net/pipermail/imagej-devel/2014-October/002280.html