Enable proxy and promise

I modified two points:

1. enable proxy via config
```javascript
var slack = new Slack("<domain>", "<token>", {proxy: "<proxy>"})
```
or use environment variable `https_proxy` or `http_proxy`.
2. enable promise
```javascript
var slack = new Slack("<domain>", "<token>", {proxy: "<proxy>"})
slack.send({channel: "#test", text: "msg", username: "muddydixon"})
  .then(function(obj){
    console.log(obj); // obj contain res and body by request
  });
```