[feature:lib] Add serialization support

[eonu]

• Adds serialization support through the following methods:
  • to_h: Serialize into a Ruby hash (with symbol keys). Accepts a boolean argument, representing whether or not to deep-serialize nested Author and Category objects (defaults to false).
  • as_json: Serialize into a Ruby hash which is also a valid JSON hash.
  • to_json: Serialize into a valid JSON string.
• Adds ATTRIBUTES constant for Paper, Author and Category entities, as a list of which attributes are available for the entity.