Support load balancing!

Right now it only works for a single hard coded project but it is a
start.  Once the LB API is settled I can update this app to create the LB directly..
Only start showing map when we know the LB is forwarding requests.

Also plumbed in uptime metric and aggregated it.  This was a false start to solving LB timing problems but is useful code regardless.
Add support for dynamically adding/removing instances.

This is a little hack as I changed the 'start instances' call from being a 'start' to really an 'ensure X number running' that will both start and stop VMs.  Things work, but it could be cleaner.
Make map display explicit.

Also robustify health checking. Add time stamp to make sure URL fetches aren't cached.  Check all LBs instead of just one.
Launch VMs from PD if the PDs exist.

Run the demos/fractal/createpds.sh script

This includes plumbing PD mount and boot support into gce.py.
Switch to CentOS as default image for fractal.
Tweak HR style a little.
Update README for fractal demo and load balancing.