Add failing test for nested addons
Implement parentAddons

When created, addons will have their `parendAddon` property set
to the addon that included them, if applicable. In addition,
the `included` hook is now passed the `parentAddon` along with
the `app`.

For addons that modified the `app` in the included hook (i.e.
preprocessors that added themeselves to the `app.registry`),
they would likely use the following pattern to determine where
to inject themselves:

    included: function(app, parentAddon) {
      var target = (parendAddon || app);
      target.registry.add('js', ...);
    }
Remove unnecessary assertion in brocfile tests

The fixture files attempt to import the result of the preprocessed
files - that essentially makes them the assertion. If the build
works, then the preprocessor worked.
Add unit tests for nested addons