Mixin the constraint API into each loaded adapter
Mixin the constraint API into each loaded adapter

* The previous approach relied on the adapters being setup, which
  may not always be true if bundler is being used. Sure, we could tell
  people to use :require => nil on adapters, but I would rather not have
  to explain that exception every-time this or related problems appear.

* Even though this was not strictly related to #1280, I am tagging it
  with the bug because it is the exact same problem as with transactions

[#1280]

datamapper/dm-constraints