callback now could be ignored

Sometimes we don't need callback, this will make things more simple
like: db.insert({"name":"kriss"});
allow object to be updated directly without upsert

var obj = {"id":1,"name":"kriss"};
storedb("users").update({"id":1}, obj);
Merge pull request #3 from KrissLiu/master

0.1.2 release