[HPT-90] Binding Paged and Page objects.

Hopefully I understood this task correctly. Basically reproduced the Hydra demo in binding pages to paged objects with two lines of code. Tested in console.

```
2.0.0-p353 :005 > b = Paged.first
 => #<Paged pid:"changeme:1", title:nil, creator:nil>
2.0.0-p353 :006 > p = Page.first
 => #<Page pid:"changeme:2", logical_number:"1", physical_number:1, text:nil>
2.0.0-p353 :007 > p.paged = b
 => #<Paged pid:"changeme:1", title:nil, creator:nil>
2.0.0-p353 :008 > p.save
 => true
2.0.0-p353 :009 > b.pages
 => [#<Page pid:"changeme:2", logical_number:"1", physical_number:1, text:nil>]
```