Installing from maven repo and added unit tests

I closed my earlier pull request in favor of this one. As in the first, I added maven integration such that you can build the plugin locally and install it in your local maven repository, which adds an alternate installation method (documented in README).

Also, I fixed the "groovy" config deprecation nag message on building.

Finally, I added some rudimentary unit tests. Additional testing would require more aggressive refactoring since many of the operations require user input or use system properties... it's a start though.