new option: Ability to display multiple pages of results

Does as many requests as specified in the `pages` option. Using the `pagination.next_url` parameter that the API returns.

Example: 
```
var feed = new Instafeed({
    pages: 10,
    limit: 1,
    get: 'tagged',
    tagName: 'thetypein',
    clientId: 'YOUR_CLIENT_ID'
});
feed.run();
```