Adding setter and __toString() methods

I've added some basic, chainable setters to ModifiersNode, UnaryOperationNode, ClassNode, and BinaryOperationNode. I also added __toString() methods to UnaryOperationNode and BinaryOperationNode.

This is really just a proof of concept, I think; ultimately I think it'd be useful to have a __toString() method on every node type exposed by Pharborist (possibly even in NodeInterface). I think it'd also be useful to have chainable setters for everything that can be changed so that the syntax tree can be modified in a structured, jQuery-like way.