[WIP]Return promise unless callback

Add a support for the Promise.

### Why
I want use the node-gitlab by Promise interface.
like:

```javascript
var gitlab = require('node-gitlab');
var client = gitlab.create({
	api: 'https://gitlab.com/api/v3',
	privateToken: 'XXX'
});

client.projects.get({id: 100})
  .then(handleResult)
  .catch(handleError);
```

### Doing
Now only RESTFulResource.prototype.list is supported.
And there is a success test case only.

If no problem I will continue.