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
actually, scratch that, we need to use a different object so that we don't mutate the default client. Could you please just add a doc comment for this var? I will even write one for you :)

```
// HttpClient is the client used by iron_go to make each http request. It is exported in case
// the client would like to modify it from the default behavior from http.DefaultClient.
HttpClient = &http.Client{}
```