Changed how the plugin loads

- Now the plugin has onEnable() and onDisable() events, and instance =
this; is done there instead of the constructor.
- Added "implements Listener" so that I can use the onLogin event.
- Changed the fileName variable containing the path to the log file.
- Changed the loginEvent to playerJoinEvent
Still doesn't work :(
Probably some problem that is SO obvious that I just don't see it.
Made the login event work

Added the registerEvent line, which, as it turnes out, is needed for
events to work and changed back from PlayerJoinEvent to
PlayerLoginEvent.
removed some problems

Moved the creation of the yaml object behind where the options are set
to what they should be.
If the input file does not exists, yaml.load returns NULL instead of an
object, so before continuing one has to be created.
Fixed some minor bugs and changed log path

Changed the path of the log file from UUID/login.yml to
logs/uuid/names.yml.
Changed some class names that where wrong.
Only assign names to a UUID if it is not known already.