Added services manager framework. Services are interfaces that specifies capabilities to be implemented by providers. Example services include economy, <insert example 2>, etc.

By: sk89q <the.sk89q@gmail.com>

paper-api/src/main/java/org/bukkit/plugin/SimplePluginManager.java

@@ -245,6 +245,7 @@ public final class SimplePluginManager implements PluginManager {
         if (plugin.isEnabled()) {
             plugin.getPluginLoader().disablePlugin(plugin);
             server.getScheduler().cancelTasks(plugin);
+            server.getServicesManager().unregisterAll(plugin);
         }
     }