Entry struct depends upon Mongo implementation

[marcossegovia]

Problem: The Entry Domain Entity is directly dependent on MongoDB implementation, as MongoDB needs a structure using the bson.ObjectId and the json comments added to be properly persisted.

Solution proposal: We need to split this an provide an independent Domain struct and a Persistence Struct that transform the current domain Object into the persistence MongoDB struct.
Also the other way around would be needed. From mongoDB struct to domain entity object to finally respond with the list of Domain Entries to the User.