Expose Container constructor

So we can use bluebird's `Promise.promisifyAll`[1*] method like so:

```javascript
var Promise = require('bluebird')
Promise.promisifyAll(Docker.Container.prototype)

// later on
Docker.createContainer(function(container) {
    container.startAsync().then(function () { ... }); // startAsync is one of the promisified methods
})
```

[1 - https://github.com/petkaantonov/bluebird/blob/master/API.md#promisepromisifyallobject-target--object-options---object](https://github.com/petkaantonov/bluebird/blob/master/API.md#promisepromisifyallobject-target--object-options---object)