Add "set but don't re-publish" flag

[jonathanvanschenck]

It would be nice to add an option to the set method which will make is so that the value isn't subsequently published over subscriptions if the value didn't actually change:

datastore.subscribe("data", console.log);
datastore.set("data", true); // prints true
datastore.set("data", true, { no_republish:true }); // prints nothing
datastore.set("data", true); // prints true
datastore.set("data", false); // prints false