remove plain objects

[bergos]

This PR removes leftovers from the plain objects idea as proposed in #104. Initial there was the idea to have plain objects without any functions for Term and Quad, but it never made it into the spec and based on some experience using implementations without plain objects, the lack of this feature doesn't cause any problems. But adding plain objects to the spec could cause a lot of problems:

• It would add much more complexity
  • Seven additional interfaces would be required (Term + subclasses, Quad)
  • Other interfaces would get more complex as they may or may not support these new interfaces
• It opens the door for a new serialization
  • People my start passing it around as JSON strings
  • Forces others to support an additional format
  • Media Type
  • ...
• JSON-LD is an existing standard for JSON + plain objects
  • flattened Quads are easy to write and parse
• It doesn't forbid the usage of plain objects for internal structures