Added API to ExperienceAPI to get the amount of XP needed for a level

src/main/java/com/gmail/nossr50/api/exceptions/InvalidFormulaTypeException.java

@@ -0,0 +1,9 @@
+package com.gmail.nossr50.api.exceptions;
+
+public class InvalidFormulaTypeException extends RuntimeException {
+    private static final long serialVersionUID = 3368670229490121886L;
+
+    public InvalidFormulaTypeException() {
+        super("That is not a valid FormulaType.");
+    }
+}