Expose service catalog to interested parties.

Added ServiceCatalogerForIdentityV2 interface to allow interested
software access to the raw service catalog.  Note that the interface
supported must necessarily expose the specific version of Openstack
Identity API version, as V2 and V3 are completely incompatible with each
other.
diff --git a/authenticate.go b/authenticate.go
index 886a6fa..076c731 100644
--- a/authenticate.go
+++ b/authenticate.go
@@ -195,3 +195,12 @@
 	})
 	return err
 }
+
+// See ServiceCatalogerForIdentityV2 interface definition for details.
+// Note that the raw slice is returend; be careful not to alter the fields of any members,
+// for other components of Gophercloud may depend upon them.
+// If this becomes a problem in the future,
+// a future revision may return a deep-copy of the service catalog instead.
+func (a *Access) V2ServiceCatalog() []CatalogEntry {
+	return a.ServiceCatalog
+}