Seems like both [read](https://github.com/mongodb/casbah/blob/master/casbah-core/src/main/scala/MongoCollection.scala#L244) and [write](https://github.com/mongodb/casbah/blob/master/casbah-core/src/main/scala/MongoCollection.scala#L795) paths are bounding the type to MongoDbObject, and casbah seems to be internally using implicit conversion from MongoDbObject to [com.mongodb.DbObject](http://api.mongodb.org/java/2.5.2/com/mongodb/DBObject.html).

On option would be to fold `MongoObjectStore` and `MongoStore` into one:
```class MongoStore[K, V](args) extends Store[MongoDbObject, MongoDbObject]```
and have separate type-specific stores?

@lanxx019 @johnynek Does this sound acceptable?