add server access from requests

[kenichi]

• add server = nil param to +Connection+ initializer
• add self to +Connection+ construction in +Server+
• add @server reader on +Connection+
• add @connection reader on +Request+
• add test for above

this gives a developer easy access to the server reactor instance from each request.

Reel::Server::HTTP.new('127.0.0.1',4567) do |c|
  c.each_request do |r|
    r.connection.server.after(3){ puts 'hi 3 seconds later' }
    r.respond :ok, 'wait for it...'
  end
end