Refactor enforcer-plugin usage

- Use <plugins> instead of <pluginManagement> to override
  parent POM version
- Upgrade to enforcer-plugin 1.3.1
- Validate Java in separate execution
Control integration tests with a profile (#417)

This puts the integration tests in a profile so that they aren't
included in the site generation for releases.
Refactor master LensKit POM

- Beef up release profile
- Subsume sonatype-oss-parent's responsibilities into our POM
- Make lenskit-packaging conditional upon being in the release profile
Add lenskit-all module (#418)
Use lenskit-all module for archetypes (#418)
Drop mldata acknowledge from archetype tests
Enable source & javadoc for snapshot deploys