POM: divide SciJava dependencies into sections
POM: update to latest SciJava software stack

The imglib2-meta project merged into imagej-common. So this commit
inverts the dependency between imagej-common and scifio: now scifio
depends on imagej-common, rather than vice versa.

The rationale is that ImgLib2 forms the base metadata-ignorant library
of N-dimensional image structures, whereas ImageJ Common provides the
metadata-rich extensions which are needed by libraries like SCIFIO.

This commit also makes the necessary code changes to adjust for the
backwards incompatibilities of the ImgLib2 2.0.1 release. In particular,
the following two things changed:

* Support for BitType images was removed, because they no longer use a
  dedicated BitArray. (Support for them could be reinstated, with some
  substantial code changes compared to how things were done before.)
* The entitiesPerPixel changed from int to Fraction.
Add DatasetIOPlugin, migrated from imagej-common

This version is kept as close as possible to the previous source code.
It will be refactored to be clean in the following commit.
DatasetIOPlugin: do not rely on deprecated methods

The DatasetService is the wrong thing to lean on for opening and saving
data here, especially since the DefaultDatasetService implementation
currently invokes SCIFIO by reflection anyway.

This change simply uses the SCIFIO-based logic as is to implement the
methods. At some point in the future, we may want a dedicated service
for this (e.g., io.scif.io.DatasetIOService), but for now, it is enough
to have the implementation in the DatasetIOPlugin directly. There will
be substantial structural code changes in the near future anyway, with
the merger of the scijava-common robust-io work.