separate Plugin from static service getters

 the plugin portion is moved to BukkitPlugin

 GenericsLib is now a static class that retrieves services from the BukkitPlugin instance
refactor GenericsLib.getLib to getPlugin
update plugin.yml file
fix exception thrown when disabling GenericsLib

Set a separate one-time flag to throw an exception if GenericsLib is accessed before it is enabled instead of using isEnabled method, which turns false when the plugin is reloaded and causes exceptions.