changes for load balancing and auto scaling

- Special cookie for load balancer to track. This takes care of the situation where user logs out and wants to log back in immediately. Should also work for deployment across zones.
- On request of a new session, JuliaBox will redirect request through the load balancer if it can't handle any more load. If an older user sesion was present on this instance, it will be backed up to s3 before redirecting.
- The redirect scheme will also be used to actively try and have fewer instances loaded to peak capacity, rather than spread load out.
- Can shut down instance for scaling down. AWS auto scaling down may not work nicely with stateful instances. This needs some more work for limiting rate of scaling down, and handling the border case where the last two instances shut down simultaneously. Optional for now.

Also:
- added Ipopt package