Implemented World.spawnBoat(), added CraftMappable interface that defines a method to get an org.bukkit.craftbukkit.CraftEntity from implementing net.minecart.server.Entity entities, changed CraftWorld.toCraftEntity() to use this new interface for boats and minecarts.

src/main/java/org/bukkit/craftbukkit/CraftMappable.java

@@ -0,0 +1,16 @@
+package org.bukkit.craftbukkit;
+
+/**
+ * Indicates that an object has a method to get its CraftBukkit-equivalent
+ * CraftEntity object from its Minecraft net.minecraft.server.Entity object.
+ * 
+ * @author sk89q
+ */
+public interface CraftMappable {
+    /**
+     * Gets the CraftEntity version.
+     * 
+     * @return
+     */
+    public CraftEntity getCraftEntity();
+}