Making it easy to activate the animal-sniffer Java5 check.

I originally attempted to define this in a profile that can be activated
from a child POM, but looking at MNG-3228
<http://jira.codehaus.org/browse/MNG-3228>, it appears that such use of
profiles aren't possible.

So I'm falling back to the <pluginManagement>, which can be activated
via the following fragment in build/plugins/plugin:

<plugin>
  <groupId>org.codehaus.mojo</groupId>
  <artifactId>animal-sniffer-maven-plugin</artifactId>
  <!-- details defined in the parent POM -->
</plugin>