Refactor, documentation improvements and cleanup.

General overview of changes:

* The base package has been changed from `org.jpokemon.pmapi` to `org.jpokemon.api`.
* Tests are now all in the same package, `org.jpokemon.api` -- there's no need to overcomplicate the directory structure.
* The `pom.xml` file has had most of the now-superfluous Maven site elements removed.

I have a few more things to add before this should be merged.