Allow any kind of object to register event listeners

There is no need to only allow Feature instances to do this. The
CommandManager already accepts any kind of object, so make this
consistent within EventDispatcher as well.
Implement most of the Database API front-end.

We now read the database configuration from mineground.yml and pass this
on to the database connection in a DatabaseConnectionParams object.
Add a query test to the DevelopmentLog feature

It's fun to see the output :-).