Refactor extract method `adapter_dependency_fulfilled?`
Add option adapters_dependencies

The RailsAdapter does not in fact depend on Rails, but only
on ActionController::Base. That observation applies to all
the currently defined SimpleTokenAuthentication adapters.
The new `adapters_dependencies` allows to ensure that
the adapters dependencies are managed in an optimal way.
Add Rails::API documentation
Add Rails::API support

Depending on the ActiveSupport::Inflection 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.

Fixes #54
Minor add version mangement recommendation to README
Add inline documentation to idempotent public method
Add inline documentation to idempotent accessor methods

Minor fix missing access policy
Add inline doc. to cryptographically critical method
Minor fix obsolete inline documentation
Minor remove obsolete dependency requirement

I forgot to remove it when I removed a deprecated Cucumber hack,
see 21ee5ed7dbb0701cffd84a12215937cdb19ae4f8 and #104
Merge branch 'add-rails-api-support'