create trusts.TokenExt response object (#29)

* create trusts.TokenExt response object

* add redelegationcount, redelegatedtrustid fields
diff --git a/MIGRATING.md b/MIGRATING.md
index 62bdf8c..aa383c9 100644
--- a/MIGRATING.md
+++ b/MIGRATING.md
@@ -21,3 +21,12 @@
   }
   res := servers.Reboot(client, server.ID, rebootOpts)
   ```
+
+# Identity
+
+## V3
+
+### Tokens
+
+* `Token.ExpiresAt` is now of type `gophercloud.JSONRFC3339Milli` instead of
+  `time.Time`