building out new RDS2 Mock.

completed:
 * create_db_instance()
 * create_option_group()

moto/rds2/urls.py

@@ -0,0 +1,10 @@
+from __future__ import unicode_literals
+from .responses import RDS2Response
+
+url_bases = [
+    "https?://rds.(.+).amazonaws.com",
+]
+
+url_paths = {
+    '{0}/$': RDS2Response().dispatch,
+}