Add method to pager so Sequel's eager can be used with pagination

Sequel can preload all matching associations by using the `eager` method. It will first load the dataset with the given query, and then it will query the database once for each one of the specified associations. For this to work, the query executing method has to be `all`, not the usual `each`, but it can still have pagination parameters as part of the query.

This change provides an `all` method to the pager, that is propagated to Sequel's dataset. It also includes `ArrayPager#all`, that returns the array range, to ensure proper compatibility with plain arrays.

Here is an usage example. It will generate 2 SQL queries, instead of 1 + N:
```ruby
# controller
@artists = paginate(Artist.eager(:albums))
```
```html
<!-- view -->
<?r @artists.all.each do |artist| ?>
  #{artist.name}
  <?r artist.albums.each do |album| ?>
    #{album.title}
  <?r end ?>
<?r end ?>
```