nsqd: make max heartbeat interval configurable
bump to 0.2.20-alpha
Merge pull request #178 from mreiferson/config_max_heartbeat_178

nsqd: configurable max heartbeat interval
nsqd: make per-client max RDY count configurable
add 'capabilities' to client IDENTIFY body; support rdy count negotiation
update go Reader to support IDENTIFY capabilities
clean up <= in protocol.md
better naming for negotiation
add test