Adding pagination to represent helper method

* All collection responses not contain a pagination key and accept pagination params `page` and `per_page`. This introduces a dependency on **kaminari**. This is baked into the `represent` method, so no changes are necessary to existing APIs to roll this out. Responses look something like this:

```
{
  data: [ ... ]
  pagination: {
    page: 1
    per_page: 25
    total_pages: 35
    total_count: 873
  }
}
```
* Defaults to 25 per page
* Fixed an AR 4.0 bug in `Napa::ActiveRecordStats`
* Added specs around `grape_extensions/grape_helpers.rb`

@jdoconnor @ckampfe @skwp @kylecrum @dshemenski