Add stacktrace to ParseError

Fixes #854. Also clean up the parse error test a little, while adding a .stack check.
Oh, I misread. Yeah, it won't be around before IE10 but I figured we don't really care about that. The code itself will run fine without a defined stack, we just need it in whatever we test in.

But that's not what we're doing to get the stack. Could you try exactly this:

```
document.write( new Error().stack );
```