multi-line exec

this fixes #12

I tried putting it on my gh-pages so you could try it out first there, but I accidentally made the PR from my gh-pages branch. either way, it works.

------------

```
var lala = 1234;
alert('lalaa');
return "lala"+lala;
> TypeError: alert is not a function
```
```
var lala = 1234;
return "lala"+lala;
> lala1234
```

```
var lala = 1234;
"lala"+lala;
> undefined
```