Add methods to convert Entity to array/json

[caleeli]

Implements #86

This PR includes:

• Add method Entity::toArray() to convert an Entity to Array.
• Add method Entity::toJSON() to convert an Entity to JSON String.
• Add method EntityCollection::toArray() to convert an EntityCollection to Array.
• Add method EntityCollection::toJSON() to convert an EntityCollection to JSON String.
• Add parameter $includeEntities in Entity::toArray() and Entity::toJSON() to include entity-type properties to the result.
• Add parameter $includeEntities in EntityCollection::toArray() and EntityCollection::toJSON() to include entity-type properties to the result.
• Unit test for Entity::toArray() and Entity::toJSON()

How to test:

• Load an Entity. eg. $entity = new User([....])
• Call to $entity->toArray() to get en Entity converted into Array.
• Call to $entity->toJSON() to get en Entity converted into JSON String.
• To include sub-entities like Gender use: $entity->toArray(true) or $entity->toJSON(true)
• To convert a collection, load a collection. e.g. new EntityCollection('User');
• Convert it to array: $collection->toArray()
• Or Convert it to JSON: $collection->toJSON()
• You could use also $includeEntities=true: $collection->toArray(true) or $collection->toJSON(true)