refactor: change attributes to "fields" for JSON API compliance

[ThorstenSuckow]

additionally, change responses and their datastructure. remove "success"-property

[ThorstenSuckow]

ToDo:

• sparse fieldsets should be considered for "attributes" selection
• meta objects should contain all data that is not described as standard with the Json API
• json to jsonApi decorators, or a converter strategy to pass to the entities