nodejs support

Add nodejs support. It can be used like
```
var code = fs.readFileSync('./code/fib.js');
var Interpreter = require('./interpreter');
var myInterpreter = new Interpreter(code);
myInterpreter.run();
console.log('value:', myInterpreter.value.data);
```
and then
`node run.js`
 where `run.js` is as above.