def environment=(environment)
      environment = environment.try(:to_sym)
      if ENVIRONMENTS.include?(environment)
        @environment = environment
      else
        raise "Unknown environment '#{environment}'. Available: #{ENVIRONMENTS.join(', ')}"
      end
    end
def environment=(environment)
      environment = environment.try(:to_sym)
      if ENVIRONMENTS.include?(environment)
        @environment = environment
      else
        raise "Unknown environment '#{environment}'. Available: #{ENVIRONMENTS.join(', ')}"
      end
    end
Configure with following options
 * :merchant_id (required)
 * :sign_key (defaults to false)
 * :environment (defaults to :development, available environments are defined in ENVIRONMENTS)
 * :proxy (a hash containing :http_proxyaddr, :http_proxyport, :http_proxyuser, :http_proxypass)