Add API to get a players experience to level (getExpToLevel). Implements BUKKIT-1906

This is the total experience one needs to gain a level.

src/main/java/org/bukkit/entity/HumanEntity.java

@@ -146,4 +146,11 @@ public interface HumanEntity extends LivingEntity, AnimalTamer, Permissible, Inv
      * @return Whether they are blocking.
      */
     public boolean isBlocking();
+
+    /**
+     * Get the total amount of experience required for the player to level
+     *
+     * @return Experience required to level up
+     */
+    public int getExpToLevel();
 }

src/test/java/org/bukkit/plugin/messaging/TestPlayer.java

@@ -764,4 +764,8 @@ public class TestPlayer implements Player {
     public boolean isValid() {
         throw new UnsupportedOperationException("Not supported yet.");
     }
+
+    public int getExpToLevel() {
+        throw new UnsupportedOperationException("Not supported yet.");
+    }
 }