Adding basic config.ru.

This does not read from config file, nor read from ARGV. Instead values are
taken from the environment vars OPENS3_PATH and OPENS3_TOKEN. Really, this is
only useful to me with testing the massive code changes

config.ru

@@ -0,0 +1,12 @@
+require 'rack'
+require 'digest'
+require File.expand_path('../lib/opens3', __FILE__)
+
+path  = ENV['OPENS3_PATH']  || File.dirname(__FILE__)
+token = ENV['OPENS3_TOKEN'] || Digest::SHA512.hexdigest('OpenS3')
+
+use Rack::Lint
+
+run OpenS3.app(path, token)
+
+