Require ActiveRecord as development dependency

Anyone who would not want to install ActiveRecord could modify
the configuration to remove it from the adapters list.

Example:

    # config/initializers/simple_token_authentication.rb

    SimpleTokenAuthentication.configure do |config|
      config.adapters = [] # was ['active_record']
    end
Do not add adapters documentation in README

Let's create antoher adapter and use the adapters interface
before making its documentatio visible.
Meanwhile, the specs do provide enough details for anyone
interested to implement adapters.