3.0.0: Element -> Node, Code generator

• Heads up: Changed transformation API - functions must accept a
  reference to the code generation process as their first parameter.
  (@thekid)
• Heads up: Added kind to UnaryExpression constructor which is
  either prefix or suffix, supporting ++$i as well as $i++.
  (@thekid)
• Merged PR #4: Add Node class replacing Element and Value - @thekid