Fix NarTestCompileMojo to exclude executable dependency
Improve NarAssemblyMojo performance by skip copying older files

NarTestCompileMojo will be failed if dependent project is an executable. The fix
is simply by-passing the dependent executable projects. NarCompileMojo has the
same logic to filter executable projects.

NarAssemblyMojo improve performance by calling
org.codehaus.plexus.util.FileUtils.copyDirectoryStructureIfModified to skip
copying files with old timestamps.
Merge branch 'master' of https://github.com/raycray/nar-maven-plugin