Scope now uses signals.SignalHandlerLoop instead of common.*

This fixes:
```
prog/app.go:310:2: undefined: "github.com/weaveworks/scope/vendor/github.com/weaveworks/weave/common".SignalHandlerLoop
prog/probe.go:309:2: undefined: "github.com/weaveworks/scope/vendor/github.com/weaveworks/weave/common".SignalHandlerLoop
```

prog/app.go

@@ -21,13 +21,13 @@ import (
 	"github.com/weaveworks/common/aws"
 	"github.com/weaveworks/common/middleware"
 	"github.com/weaveworks/common/network"
+	"github.com/weaveworks/common/signals"
 	"github.com/weaveworks/go-checkpoint"
 	"github.com/weaveworks/scope/app"
 	"github.com/weaveworks/scope/app/multitenant"
 	"github.com/weaveworks/scope/common/weave"
 	"github.com/weaveworks/scope/common/xfer"
 	"github.com/weaveworks/scope/probe/docker"
-	"github.com/weaveworks/weave/common"
 )
 
 const (
@@ -307,7 +307,7 @@ func appMain(flags appFlags) {
 	}()
 
 	// block until INT/TERM
-	common.SignalHandlerLoop()
+	signals.SignalHandlerLoop()
 	// stop listening, wait for any active connections to finish
 	server.Stop(flags.stopTimeout)
 	<-server.StopChan()

prog/probe.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/weaveworks/common/network"
 	"github.com/weaveworks/common/sanitize"
+	"github.com/weaveworks/common/signals"
 	"github.com/weaveworks/go-checkpoint"
 	"github.com/weaveworks/scope/common/hostname"
 	"github.com/weaveworks/scope/common/weave"
@@ -32,7 +33,6 @@ import (
 	"github.com/weaveworks/scope/probe/plugins"
 	"github.com/weaveworks/scope/probe/process"
 	"github.com/weaveworks/scope/report"
-	"github.com/weaveworks/weave/common"
 )
 
 const (
@@ -306,5 +306,5 @@ func probeMain(flags probeFlags, targets []appclient.Target) {
 	p.Start()
 	defer p.Stop()
 
-	common.SignalHandlerLoop()
+	signals.SignalHandlerLoop()
 }