Add Snapshot struct and corresponding Export/Import functions.
Add Snapshot struct and corresponding Export/Import functions.

This is useful for serializing Histograms and then reconstructing them.
This also adds an Equals method for comparing Histograms for equality.

tylertreat/hdrhistogram,codahale/hdrhistogram,ahothan/hdrhistogram,zenazn/hdrhistogram,ahothan/hdrhistogram