Require missing dependency ActionPack

... which provides ActionController

See https://github.com/rails/rails/tree/4-1-stable/actionpack/lib
Refactor ensure adapters are loaded only when needed

ActiveRecord is a runtime dependency; the ActiveRecordAdapter
will be loaded as usual and this commit introduces no changes
in the gem behaviour.
Minor add adapter interface spec

The shared specs for the 'adapter' interface provide a better
help if they spot the methods which miss an override.

Adding this spec makes the shared specs no longer usable with
the `Adapter` module itself. Some more caution will be needed
to ensure both the module and the shared specs stay in sync, but
the module should change less often than new adapters will be added.
Minor refactor tidy up TokenAuthenticationHandler

Fix circular dependency (!)
Minor fix harmless variables shadowing in dynamically defined methods

Remove forgotten hack related to the deprecated Cucumber scenarios