Add custom serializer configuration

Add new config option `:serializer` for specifying a serializer for the redis
session storage.

The possible values are:

* `:json` - serialize cookie values with `JSON`
* `:marshal` - serialize cookie values with `Marshal`
* `:hybrid` - transparently migrate existing `Marshal` cookie values to `JSON`

Thanks to Łukasz Sarnacki for the work on the rails cookie storage json
serializer.