Cleanup

This started as just making sure the version number only has to be maintained in `pom.xml`, but I found a couple other things I could clean up.

* Added a section to `pom.xml` that will automatically copy the version number into `plugin.yml` on every build.
* Removed unnecessary (and unconventional) `main.java` from the package name
* Optimized the imports with IntelliJ. It removed unused imports and grouped similar imports together.

I'm curious, which IDE do you use that it produced that `main.java` prefix?