Points awarded on destroying a model

This is an initial attempt to solve https://github.com/tute/merit/issues/143 by serialising the model into the DB.

I had trouble getting JSON serialisation to work as there appeared to be a bug of some sort, but leaving the serialisation as the default YAML works fine.

I have no idea if this will work for Mongoid as I am not familiar with that system at all. I have added the field definition for the action model, but don't know if the serialisation and reanimation will work.
This is backwards compatible change, as we need a migration on the database. But I want the next release to indeed be `2.1.0`, so we should show a nice warning message for users that they should run the migration if they want this feature, instead of failing because the column doesn't exist. This is a new feature that works provided that migration, and on a future major version release we can remove that warning message.

We can do it deluxe, adding a rake merit:upgrade task that adds this specific migration.

Thanks for your good work!