api: Add new ListenBucketNotificationHandler. (#2336)

This API is precursor before implementing `minio lambda` and `mc` continous replication.

This new api is an extention to BucketNofication APIs.

// Request
```
GET /bucket?notificationARN=arn:minio:lambda:us-east-1:10:minio HTTP/1.1
...
...
```

// Response
```

{"Records": ...}
...
...
...
{"Records": ...}
```

pkg/sys/stats_test.go

@@ -0,0 +1,16 @@
+// +build linux darwin windows
+
+package sys
+
+import "testing"
+
+// Test get stats result.
+func TestGetStats(t *testing.T) {
+	stats, err := GetStats()
+	if err != nil {
+		t.Errorf("Tests: Expected `nil`, Got %s", err)
+	}
+	if stats.TotalRAM == 0 {
+		t.Errorf("Tests: Expected `n > 0`, Got %d", stats.TotalRAM)
+	}
+}