Change use of String permissions to the permission enums

src/main/java/fr/xephi/authme/permission/PermissionNode.java

@@ -0,0 +1,11 @@
+package fr.xephi.authme.permission;
+
+/**
+ * Common interface for AuthMe permission nodes.
+ */
+public interface PermissionNode {
+
+    /** Return the node of the permission, e.g. "authme.unregister". */
+    String getNode();
+
+}