Added the ability to save Symbols to MongoDB

With this change now MongoDB is responsible of saving Symbols (Because MongoDB can save Symbols). Before this, i think the Symbols were saved in MongoDB as a '__marshaled__'.
Gems should always restrict the versions of their dependencies they use. I added the 1.0 restriction specifically because when moped 2.0 was released, it broke this gem. Is saving symbols specific to moped 2+?