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.
Raising an exception is not a warning! :wink: We should log it into `Rails.logger`, and continue normal operations.