[ADDED] Ability to get the server's HTTP Handler

This will allow applications (for instance NATS Streaming Server)
to add new endpoints to the NATS http server.

Resolves #480

server/pse/pse_darwin.go

@@ -3,18 +3,18 @@
 package pse
 
 import (
-	"errors"
 	"fmt"
 	"os"
 	"os/exec"
 )
 
+// ProcUsage returns CPU usage
 func ProcUsage(pcpu *float64, rss, vss *int64) error {
 	pidStr := fmt.Sprintf("%d", os.Getpid())
 	out, err := exec.Command("ps", "o", "pcpu=,rss=,vsz=", "-p", pidStr).Output()
 	if err != nil {
 		*rss, *vss = -1, -1
-		return errors.New(fmt.Sprintf("ps call failed:%v", err))
+		return fmt.Errorf("ps call failed:%v", err)
 	}
 	fmt.Sscanf(string(out), "%f %d %d", pcpu, rss, vss)
 	*rss *= 1024 // 1k blocks, want bytes.