Use Heapster as an in-built monitoring solution for Kubernetes in GCE.

Users will have an option to enable it when they setup their cluster (kube-up).

cc @thockin @vmarmol @rjnagal 
Hmmm I'm torn. I don't think we should include these instructions here (they are in the heapster repo and we can point people there). Maybe just have the ones for how to access heapster? Here we should probably put the instructions of how to make kube-up enable heapster.