auto scaling improvements, config tweaks, added curl

auto scaling improvements:
- instances with older AMI relinquish cluster leadership proactively
- consider loads of instances with most recent AMI only for scaling up as others stop accepting new requests
- more aggressively release machines loaded less than half

configuration changes:
- nginx configured to allow larger uploads without buffering to disk (for large notebooks)
- notebook websocket protocol configurable
- EBS optional in mounting script

make curl available:
- fixes #157