Pretty massive 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.
* The package structure has been simplified, and paths are now simpler and shorter; see detail below.

New package structure.

* As a general rule, anything that deserves a manager gets its own package.
* I moved everything out of the `pokemon` package into the base package.
* Package names are now plural (items, moves, abilities, natures, types, trainers).
* New: `org.jpokemon.api.classic` package contains "implementation-specific" code (i.e. ClassicTypes, ClassicNatures, but also StatusAilment).
* Removed: `util` --- contents folded into `classic` for now, although it is possible that things may belong here in the future.


I have a few more things to add before this should be merged.