Only calls rehydrate if old and new values differ (#16)

See #9 

IE 11 seems to get stuck in an infinite loop of `storage` events sometimes. By only rehydrating if the old and new values aren't the same, this problems seems to be fixed *yaay*

index.js

@@ -14,6 +14,7 @@ module.exports = function(persistor, config){
  var keyspace = e.key.substr(keyPrefix.length)
  if(whitelist && whitelist.indexOf(keyspace) === -1){ return }
  if(blacklist && blacklist.indexOf(keyspace) !== -1){ return }
+ if(e.oldValue === e.newValue){ return }
 
  var statePartial = {}
  statePartial[keyspace] = e.newValue