Add support for Kaminari, a new configuration class and a more modern total count header

Adds additional support for Kaminari.
Adds a new way to configure the gem via initializer:

```ruby
# config/initializers/api_pagination_headers.rb
ApiPaginationHeaders.configure do |config|
  config.total_count_header = 'X-Total-Count'
end
```

The default total count header is now `Total-Count` and as a result I have bumped the version to 2.0. The "X-" prefix for headers has been deprecated, see [RFC 6648](http://tools.ietf.org/html/rfc6648).