Extract local variable client to package level

This change allows to setup parameters of http.Client

```
q := mq.New("test")
api.HttpClient.Timeout = 20 * time.Second
q.PushString("Test")
```

Discussion available here: https://github.com/iron-io/iron_go/issues/28
nitpick, but `http.DefaultClient`, even though entirely equivalent I think gives a better clue to what we're doing here.