WIP Node.js port

Hi Gravicappa,

This is a work in progress node.js port of Shen.

##### The following files are added:

* `shen-node.js` program entry
* `runtime-node/lib.js` runtime functions and hacks
* `package.json` required package file

##### The following file is changed:

* `.gitignore`: ignore `node_modules` directory

##### How to test:

* Clone the repo
* Install node.js on your platform. Make sure you can use `npm` and `node` commands
* `npm install`
* `./shen-node.js` or `node shen-node.js`

##### Bugs

* Sometimes entering a partially completed expression will cause endless recursive calls. This is caused by shen.js keeping calling `repl_read_byte` despite getting -1 responses. I think it's a problem with the shen.js file. To test this bug, type `(str [enter]` in the repl.
* Transpile/execute shen.js do not work. I haven't figured out how they work in shen.js yet.

Please test out the code and report any problems so I can fix them.

Regards,
xkxx