Add Stats collector service for Scaler.

 Currently reports list of nodes with capacity - memory and cpu.

Periodically collects and reports  mean, max, and 90p usage stats for memory and cpu.
Google says its fine.

Its also from golang doc: 
http://golang.org/doc/effective_go.html#for
for key := range m {
    if key.expired() {
        delete(m, key)
    }
}