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/acceptance/12-update-server.go b/acceptance/12-update-server.go
new file mode 100644
index 0000000..203b37d
--- /dev/null
+++ b/acceptance/12-update-server.go
@@ -0,0 +1,44 @@
+package main
+
+import (
+	"fmt"
+	"flag"
+	"github.com/rackspace/gophercloud"
+)
+
+var quiet = flag.Bool("quiet", false, "Quiet mode, for acceptance testing.  $? still indicates errors though.")
+
+func main() {
+	flag.Parse()
+	withIdentity(false, func(acc gophercloud.AccessProvider) {
+		withServerApi(acc, func(servers gophercloud.CloudServersProvider) {
+			log("Creating server")
+			id, err := createServer(servers, "", "", "", "")
+			if err != nil {
+				panic(err)
+			}
+			waitForServerState(servers, id, "ACTIVE")
+			defer servers.DeleteServerById(id)
+
+			log("Updating name of server")
+			newName := randomString("ACPTTEST", 32)
+			newDetails, err := servers.UpdateServer(id, gophercloud.NewServerSettings{
+				Name: newName,
+			})
+			if err != nil {
+				panic(err)
+			}
+			if newDetails.Name != newName {
+				panic("Name change didn't appear to take")
+			}
+
+			log("Done")
+		})
+	})
+}
+
+func log(s string) {
+	if !*quiet {
+		fmt.Println(s)
+	}
+}
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
 
diff --git a/servers.go b/servers.go
index 6b97f05..16d82c5 100644
--- a/servers.go
+++ b/servers.go
@@ -247,6 +247,26 @@
 	})
 }
 
+// See the CloudServersProvider interface for details
+func (gsp *genericServersProvider) UpdateServer(id string, changes NewServerSettings) (*Server, error) {
+	var svr *Server
+	err := gsp.context.WithReauth(gsp.access, func() error {
+		url := fmt.Sprintf("%s/servers/%s", gsp.endpoint, id)
+		return perigee.Put(url, perigee.Options{
+			ReqBody: &struct{
+				Server NewServerSettings `json:"server"`
+			}{changes},
+			MoreHeaders: map[string]string{
+				"X-Auth-Token": gsp.access.AuthToken(),
+			},
+			Results: &struct{
+				Server **Server `json:"server"`
+			}{&svr},
+		})
+	})
+	return svr, err
+}
+
 // RaxBandwidth provides measurement of server bandwidth consumed over a given audit interval.
 type RaxBandwidth struct {
 	AuditPeriodEnd    string `json:"audit_period_end"`
@@ -372,6 +392,14 @@
 	OsExtStsVmState    string         `json:"OS-EXT-STS:vm_state"`
 }
 
+// NewServerSettings structures record those fields of the Server structure to change
+// when updating a server (see UpdateServer method).
+type NewServerSettings struct {
+	Name string `json:"name,omitempty"`
+	AccessIPv4 string `json:"accessIPv4,omitempty"`
+	AccessIPv6 string `json:"accessIPv6,omitempty"`
+}
+
 // NewServer structures are used for both requests and responses.
 // The fields discussed below are relevent for server-creation purposes.
 //