Features/json adapter

First draft of JSON serialization and deserialization according to DotAAS, section 5.5 (see #21 (closed)). Basic implementation is finished, but some things are left to do:

• Add thorough testing (current simple test reaches only 40% coverage)
• Further improve testing:
  • Add fail tests for deserialization
  • Add example with None-Values
• Verify license of JSON Schema file (If it is not guaranteed to be provided under a free license, we need to rebase the branch to remove it from the history.)
• Merge master and adapt serialization/deserialization for changes on master branch:
  • Reference vs. AASReference,
  • base classes of View,
  • add license header
  • use model.KEY_ELEMENTS_CLASSES instead of copy in aas.adapter.json.json_deserialization.
• Fix insertion of modelType for non-Referable classes in JSON serialization