avoid strict standard errors by dropping abstract static

lib/DbMapping.php

@@ -108,7 +108,9 @@ abstract class DbMapping
      * @param int $id
      * @return DbMapping
      */
-    abstract public static function find($id);
+    public static function find($id) {
+        throw new RuntimeException("This method must be implemented in child classes.");
+    }
 
     /**
      * Return all the public attributes in an array;