add cache to speed up multi-module projects

Add a simple cache so that multi-module projects don't spend time re-determining common dependencies. Testing against a complex project with 10 modules shows a significant speed up:

    mvn -o com.ning.maven.plugins:maven-duplicate-finder-plugin:1.0.4:check
    …
    [INFO] Total time: 23.180s

vs:

    mvn -o com.ning.maven.plugins:maven-duplicate-finder-plugin:1.0.5-SNAPSHOT:check
    …
    [INFO] Total time: 4.455s