Some more backwards incompatible changes (minor though), also a ton of small cleanup.

onPluginEnable(PluginEvent event)   -> onPluginEnable(PluginEnableEvent event)
onPluginDisable(PluginEvent event)  -> onPluginDisable(PluginDisableEvent event)
onVehicleUpdate(VehicleEvent event) -> onVehicleUpdate(VehicleUpdateEvent event)
onWorldSave(WorldEvent event)       -> onWorldSave(WorldSaveEvent event)
onWorldLoad(WorldEvent event)       -> onWorldLoad(WorldLoadEvent event)

src/main/java/org/bukkit/event/server/PluginEnableEvent.java

@@ -0,0 +1,9 @@
+package org.bukkit.event.server;
+
+import org.bukkit.plugin.Plugin;
+
+public class PluginEnableEvent extends PluginEvent {
+    public PluginEnableEvent(Plugin plugin) {
+        super(Type.PLUGIN_ENABLE, plugin);
+    }
+}