[mongo-livedata] Add support for bulk insertion of an array of documents

I noticed that the `mongo-livedata` package does not support the bulk insertion of models when `insert` is passed an array of documents. This is a standard mongodb feature as of `2.2`. This pull request adds support for this feature.

The API looks like this...

```javascript
var docIds = People.insert([ { name: 'Alex' }, { name: 'Claire' } ]);
// docIds will be an array with the ids of each document
// ie... ['fvWyGta6o9SGtTMFa', 'gJEFMgaJSFeeTqCPg']
```

I thought returning a list of IDs for the inserted documents made sense, let me know if you would prefer something else.

Cheers