Mavenize the project

This moves the source code and resources to follow the standard Maven
packaging structure, and adds a pom file.

Maven is not required to build, though you would have to adjust some
things to continue building the old way. If you're willing to install
Maven, you can now simple use "mvn package" to generate a jar.

I also auto-versioned plugin.yml to match the pom version, but you can
undo that if you want.

I'd be willing to host you on my Maven repo if you'd like!