Improve handling of optional artifact uploads, including tests

[knightlabs]

I'm not sure this is really the most effective mechanism for doing this, but I've got two needs: primarily, the ability to upload test-jars and secondarily, the ability to make sources, tests and javadoc optional.

I actually changed the defaults to not include any of the three, but open to suggestions as to why to why they were automatically included originally.

[bmuschko]

Thanks for the pull request! I am going to look at this sometime this week.

[bmuschko]

Thanks for your contribution. I merged the pull request.

I changed some of the defaults. With the previous version of the plugin, tasks for Javadoc and sources generation were automatically added to the projects. I want to stick with this convention to make the new version backward compatible.