Added minecart classes, vehicle class, and the ability to spawn minecarts. StorageMinecart needs a getInventory(), but that is waiting on the addition of inventory code.

src/main/java/org/bukkit/PoweredMinecart.java

@@ -0,0 +1,10 @@
+package org.bukkit;
+
+/**
+ * Represents a powered minecart.
+ * 
+ * @author sk89q
+ */
+public interface PoweredMinecart extends Minecart {
+
+}