Implement search path for JavaAssets.

This is an alternative to pull request https://github.com/threerings/playn/pull/41

Notes on that:
* We don't need the directories in the config or assets ctor, since tha java platform exposes the
  JavaAssets directly. This is more in keeping with the approach used by setPathPrefix.
* The pathPrefix is only applied to the classloader attempt. I figure this makes more sense, so
  extra file system directories may or may not include the prefix.
* requireResource now throws an IOException, since it is doing more complex I/O
* This also addresses getAssetStream, which the pull request did not.
* requireResource does *not* expect the path prefix to already be preprended. This makes it match
  getAssetStream, which is used by registerFont. subclassers beware!

Also added some comments.