pomp / TODO

 * write docs about BaseDownloader as recursive downloader
   implement new `queued downloader`, witch must get requests from external queue,
   and put next request to it (needed for distributed systems)
 * option to limit `request per host`
 * downloader middlewares can return more than one reqeust/response
 * plain threads support and async behavior
 * gevent support
 * tornado support
 * tulip support ?
 * GAE support