v0.4.0

Release with significant improvements:

• Switched from openresty bundled cjson to lua-resty-libcjson. First and only reason — possibility to specify how to encode empty lua tables (sometimes it should be array, sometimes — object)
• Mostly rewrote pub/sub part:
  • Each topic have one subscription id aside from clients
  • More clean publishing
  • Less redis keys and logic