Variable scope differs between Javascript and webppl

Consider this example:

```
if (true){
  var x = 1;
} else {
  var x = 2;
}
x
```

In Javascript, `if` blocks don't introduce new scope, so `x` is 1. In webppl, this results in

```
Uncaught ReferenceError: x is not defined
```

This isn't necessarily a problem (in this example, can use ternary operator), but we should document differences like this at some point.
