Fix #3: Add plugin descriptor to main sourceset.

  - Adding to the main sourceset instead of directly to the jar file
    makes the plugin descriptor available in the test classpath so the
    plugin may be applied in tests, etc.

This seemed like the best way to accomplish this, but let me know if there is a better way.
I think it may be good to introduce a static final var for the `plugindev` directory; where do you think makes the most sense to put it?