Revised Javadoc of `ClearSystemProperties`.

Added some additional information and simplified the example. Hopefully
it makes it easier to understand the rule.
Revised Javadoc of `RestoreSystemProperties`.

Add some additional information and simplified the example. Hopefully it
makes it easier to understand the rule.
More to TestWatcher from ExternalResource.
Add log-on-failure mode with tests.
Merge branch 'master' of github.com:binkley/system-rules