Provide a way to skip a project's tests if nothing has changed

Surefire should have an "incremental" mode, perhaps enabled by default.

We should check the timestamps of every element of the classpath (and every class in any classpath folder) and touch a file in "target" with that timestamp. If none of those files have been updated, skip tests for the current project.

It definitely needs to be possible to turn this feature off, though, because sometimes you need to re-run tests based on external resources not on the classpath: deployed web servers, database schema changes, etc.