Add promote_singleton_appium_methods(main_module)

module App
  module Splash
    class << self
      def login
        'login'
      end
    end
  end
end

module Kernel
  def splash
    App::Splash
  end
end

Appium.promote_singleton_appium_methods(App)

This enables accessing $driver methods within the App::Splash page object.

Fix #73

Raise on nil driver