Add simpler API for using the scheduler. Adds BUKKIT-836

The new methods return the actual task that gets created from the
scheduler. They are also named such that auto-complete puts the
asynchronous methods after the normal ones. These two additions are
simply semantic.

Tasks now have a method to cancel themselves using their task id. This
is provided as a convenience.

A new class called SimpleRunnable was added. It is an abstract Runnable
such that anonymous classes may subclass it. It provides six convenience
methods for scheduling as appropriate. It also provides a cancel method
for convenience. The functionality of SimpleRunnable only stores an
integer representing the task id. A SimpleRunnable can only be scheduled
once; attempting to reschedule results in IllegalStateException.

src/main/java/org/bukkit/scheduler/BukkitTask.java

@@ -28,4 +28,9 @@ public interface BukkitTask {
      * @return true if the task is run by main thread
      */
     public boolean isSync();
+
+    /**
+     * Will attempt to cancel this task
+     */
+    public void cancel();
 }