support/spec for bundles

i.e. `build.scripts()` can return an hash of strings based on the "bundle name".

``` json
{
  "boot": "",
  "users": ""
}
```

the `tree` can be split into multiple bundles based on some https://github.com/component/bundler.js algorithm.

probably should turn this into a spec, then make sure other libs can handle it.
