Added functions to ExperienceAPI for use with offline players.

Fixes #836

src/main/java/com/gmail/nossr50/api/InvalidPlayerException.java

@@ -0,0 +1,9 @@
+package com.gmail.nossr50.api;
+
+public class InvalidPlayerException extends RuntimeException {
+    private static final long serialVersionUID = 907213002618581385L;
+
+    public InvalidPlayerException() {
+        super("That player does not exist in the database.");
+    }
+}