Formatter and license headers

I've added the plugin we use for ead/mokap to get a consistent code style, and also other that adds a license header automatically to each source file.

To keep code clean styled, we have to invoke `mvn clean install` always before submitting the code.

Btw, I've initially selected Apache2.0 license, but we could pick any other license we may like. Opinions?

Closes #19