Use key "username" (not "user") for loginWithUsername

When trying to login with the above function, I get (in Meteor 0.8.3):

```js
{ error: 400,
  reason: 'Match failed',
  message: 'Match failed [400]',
  errorType: 'Meteor.Error' }
```

correcting the above fixes the problem, not sure if this changed in Meteor recently.