Don't negotiate connection tune values down to 0
Now known to be used in production

A kind soul wrote to let me know ..
Merge branch 'negotiate' of github.com:cloudamqp/amqp.node into pr84-tune
Remove erroneous comment

And tweak corrected code into house style
saner defaults

channelMax: 65535 channels
frameMax: 131072 bytes
heartbeat: 580s
don't change default heartbeat, use server's default