Add update server capability.

This requires an updated version of Perigee, for it lacked a Put()
method.  Make sure you "go get -u github.com/racker/perigee" prior to
testing, or you'll get method not defined errors.
diff --git a/interfaces.go b/interfaces.go
index 2bb8ac5..8e02fc3 100644
--- a/interfaces.go
+++ b/interfaces.go
@@ -38,6 +38,7 @@
 	RebootServer(id string, hard bool) error
 	RescueServer(id string) (string, error)
 	UnrescueServer(id string) error
+	UpdateServer(id string, newValues NewServerSettings) (*Server, error)
 
   // Images