Transform object into hash

This code part need review. @yaf @sem2173 @akhardya ?

Had some difficulties to grab the created object into the `.to_hash` method. I probably didn't do the best way of doing it.

I've [skiped the next test for DB](https://github.com/benoittgt/revue/blob/hash_features/spec/revuelog_spec.rb#L15-L17) but @sem2173 and @akhardya feel free to edit the test and plug Mongodb.