Fix ListServers to yield full details.

Also, provide ListServersLinksOnly() if you want to retain the older
behavior.  LinksOnly variant isn't terribly useful though, as it
provides virtually _no_ useful information other than the mere existence
of a server.

Fixes #45.
diff --git a/interfaces.go b/interfaces.go
index b73a118..4342553 100644
--- a/interfaces.go
+++ b/interfaces.go
@@ -27,6 +27,7 @@
   // Servers
 
 	ListServers() ([]Server, error)
+  ListServersLinksOnly() ([]Server, error)
 	ServerById(id string) (*Server, error)
 	CreateServer(ns NewServer) (*NewServer, error)
 	DeleteServerById(id string) error