Add test to notice when copyrights need updating

[Michael Terry]

Right now, we update copyright headers haphazardly. But it should be easy to automate.

A line like this should get all the files in bzr that updated this year:

bzr status --short --versioned -r date:2016-01-01.. | grep -v ^-D | grep -v '^R '

Then we use one of the various copyright update tools in the archive to check/update those files. (Something like copyright-update, licensecheck, license-reconcile, or licensing. I haven't surveyed them for which is best for us.)

So we could add a test for this and a convenience script to automatically update the files that are out of date.