Filter non-jar artifacts when packaging libs in Gradle plugin

Previously, the Gradle plugin would package all of a project's
dependencies in the jar's lib directory, irrespective of each
dependency's type. This led to non-jar artifacts being packaged in
the lib directory where only jar dependencies are expected. See #334
for an example failure.

This commit updates the Gradle plugin such that it only packages
dependencies of type jar, ejb, ejb-client, test-jar, or bundle. This
brings the Gradle plugin into line with the Maven plugin.

Fixes #334.