Add adapters dependencies registration

Add `adapters_dependencies` configuration option
Refactor extract method `adapter_dependncy_fulfilled?`

The Rails::API dependencies can't be read from the adapter name
as the MongoidAdapter, ActiveRecordAdapter or RailsAdapter dependencies could.
Moreover, that last adapter does in fact **not** depend on Rails, but on
ActionController::Base instead.

The new `adapters_dependencies` configuration option allows to register
properly the adapters dependencies.

Besides, depending on the ActiveSupport::Inflectior settings -namely
if the 'API' acronym is not defined- Rails will attempt to require
the SimpleTokenAuthentication::Adapters::RailsApiAdapter constant (lowercase).
To avoid forcing the acronym definition, the adapters responds to both names.