Updated node.js port

Good evening Gravicappa,

This pull request patches the node.js port, adding support for compilation & execution of source files. 

The following files are changed:

* shen-node.js - adding compilation & execution of source files
* runtime/lib.js - updated Shen.io functions
* package.json - added shen-js command
* README.md - documentations

Bugs & WIP:

* The return values in REPL are now plain JavaScript. Could you advise me which function to use to prettyprint Shen variables?
* Stack traces are unhelpful when executing source files.

Regards,
xkxx