Fix relationship set up (inspired by the rack-core-data project)
Add a new Relationship class to encapsulate the relationship logic
Add endpoints for the url access of relationship objects
Merge remote-tracking branch 'dulaccc/relationship-url-following'

* dulaccc/relationship-url-following:
  Add endpoints for the url access of relationship objects
  Add a new Relationship class to encapsulate the relationship logic
  Fix relationship set up (inspired by the rack-core-data project)

Conflicts:
	lib/rack/scaffold.rb
	lib/rack/scaffold/adapters.rb
	lib/rack/scaffold/adapters/core_data.rb
	lib/rack/scaffold/adapters/sequel.rb