Rack::Maintenance module

Added a simple middleware for serving maintenance prompts. 

By default middleware will look into current working directory for file with filename ".maintenance".
Request will be passed next if no file found.

Simpliest example:

    use Rack::Maintenance

Set custom directory:

    use Rack::Maintenance, '/path/to/dir'

Customize default maintenance prompt:
  
    use Rack::Maintenance do
      "We're doing some stuff. Come back later."
    end

If you want to use timestamps in your prompt just put a valid time string
into your .maintenance file:

    File.open('.maintenance', 'w') { |f| f.write(Time.now + 3600) }

and then use it in your prompt:

    use Rack::Maintenance do |t|
      "We're doing some stuff since #{t[:since} will be done at #{t[:until]}."
    end