Merge with upstream

- Removed cascadae from dependencies;
- Added azDHT (http://wiki.vuze.com/w/Distributed_hash_table);
- Added PEX (not yet ready. It is read-only and do not check peers before adding);
- Added debug messages into endgame-module (it stucks sometimes);
- Refactored metadata downloader (for magnet-links);
- UPNP works with Cowboy 0.8.0.

TODO:
- a mainline DHT node tracks peers for added torrents only. It must allow to publish other torrents too;
- a table for tracking progress of connected peers.