Move to Gradle, fix package names, reduce jar size

[AlessioGr]

I have spent 6 hours trying to build this plugin in Maven, but it failed every time because of seemingly random errors which made no sense. Thus, I switched it over to gradle, and it worked just fine.

This decreases the final size of the jar by 30kb (for some reason, the source was included in the last one) and it makes builds a lot faster.

The plotsquared dependency was updated as well.

I have also corrected the package names. They were all lowercase, which did not compile for me.