Add Persistable base class for persisting data to Parse

This adds a `Persistable` base class that `User` and `Story` extend from. You can use `Persistable.dataSource` for persisting changes to Parse. Also, you can use `Persistable.find(id)` for finding a record in Parse. For instance, call `User.find("some_id")` to find a user record.

/cc @snekkalapudi @rvharsha