Add support for one-step increments to the Redis engine.

[alexkiro]

I.e. in the same way we added support for the MySQL engine in #23 but for the Redis engine.

In the current way the records are encoded in the Redis engine, one-step increments are not possible. We'll need to change the records from string to hashes, and also provide a way to migrate the database.

The existing migration script can be used. A simple check should be performed when starting the pyzord server looking for a "version" entry in the database and checking if it matches the current implementation. If not then a error message should be displayed with instructions on migrating the database.