Add/helpers

/cc @ianstormtaylor we gotta create a `.stub` yeah?
all this can probably simplified if we're doing multiple lines:

```js
assert(
  spy.got.apply(spy, args), 
  fmt('Expected "%s" to be called with "%o", but it was called with "%o".',
  spy.name, args, spy.args[0])
);
```

or something