Add non-enumerable parent reference to each node. Fixes #19

This is kinda a hack, not sure if it is a good idea or not. Basically this scans through the whole AST, looking for nodes that have the `type` property set, and adding a non-enumerable `parent` property to the node. This is done after the whole AST is parsed, so it doesn't do any good for things like errors that required tracking the parent as it is parsing.

Also, not sure if it is a good idea to simply traverse all the keys of the AST.

Basically this is a very simple solution, but might not be the best. Feedback needed.