Brand extended Express apps with exports instead of true

[ericf]

This provides a more useful value for the "brand" applied to extended Express apps, the module object itself.

[ericf]

@caridy Do you like this approach instead of simply branding the app's with the value true? I think this will make it easier to compare which version of the module is actually extending the app.

[caridy]

LGTM. most of the components that extends express are already doing that. In the case of express-yui I'm storing a reference to the class used to create app.yui member since it is not exposed thru the module directly, but I might change that to complain with this.