Specify the collection for a model

ref #7

It'd be helpful to be able to set the collection for a given model, I thought this way could work & wasn't too disruptive:

```js
var mongo = require('modella-mongo')('localhost/myapp')
var User = model('User')

// db.User
User.use(mongo)

// db.app_users
User.use(mongo('app_users'))
```

Thoughts?