Got the buildnumber-maven-plugin working.  The trick was to set (override) the <scm> property of the Maven build.  This begins to address issue #59.
Add information about the build revision, branch, and time to the jar manifest.
Move buildnumber plugin information to the top-level POM so that all sub-projects get the build information.
Add build information to war files.
Remove outdated scm information from student archetype POM.
Merge pull request #89 from DavidWhitlock/issue-59-git-revision-in-manifest

Add information about the build (time, revision, and branch) to jar and war manifests.

In order to address issue #59, I used the buildnumber-maven-plugin to determine the time at which the Maven build occurred and the git branch and revision from which the code was built. That build-related information is included in the MANIFEST.MF entry in jar and war files. I verified that Maven projects created from the archetypes (student-archetype, for instance) still build even though they are not under revision control.