Simplify pom-imglib2 by extending pom-imagej

This inherits the shared configuration of pom-imagej, including
managed versions of the imagej, imglib2 and scifio organizations.

For details of the decision making progress which led to this, see:
http://imagej.net/pipermail/imagej-devel/2014-October/002280.html
Merge branch 'imagej-is-da-bom'

For discussion and rationale, see:
http://imagej.net/pipermail/imagej-devel/2014-October/002280.html