Provide an import summary (see #12506).

This PR is an initial attempt at fixing http://trac.openmicroscopy.org.uk/ome/ticket/12506. It adds 3 summary lines after the CLI import finishes. The output should be self-descriptive.

To test:
- do a CLI import of single-image/MIF datasets/folders and verify that the numbers make sense.

@sbesson: Is there any good place for some integration tests?