minor updates on Node function

- removed unneeded commas
- changed `throw '...'` to `throw new Error('...')`
changed throw '..' to throw new Error('..') in Node
added code comment in Node_Blank
added tests for Node_Blank
added test for Node_Concrete
added test for Node_Fluid
fixed getLexicalForm method of LiteralLabel

- before getLexicalForm did not return a String object in all cases
- added explicit .toString() call of the return value if it is not a
  String instance
- adjusted tests (that previously tested this bug)
added tests for Node_Literal
added tests for Node_Uri
added tests for Node_Variable
added tests for Var
added tests for Node
removed unneeded commas