configuration changes

- nginx configured to allow larger uploads without buffering to disk (for large notebooks)
- notebook websocket protocol configurable
auto scaling improvements

- instances with older amis relinquish cluster leadership proactively
- improvements to auto-scaling logic
- also made ebs optional in mounting script
make curl available. fixes #157
Merge pull request #159 from tanmaykm/tanmaykm

auto scaling improvements, config tweaks, added curl