MurmurHash : Add std::hash specialization, for use in std::unordered_map

[danieldresser-ie]

John requested I add this, when I needed to hack around it for a Gaffer PR. That Gaffer PR is now using an LRUCache instead of an std::unordered_map, so it wasn't needed there, but I might as well still add this.

[johnhaddon]

Thanks Daniel - this is good timing as I currently have a use case for this in my Dispatcher work.