Add Application Architecture category in demos, u2020 and u2020-mortar apps

New category in demos - Application Architecture. It covers demo apps that show how to build application code structure using several techniques and open-source libraries.
And few examples:
* [u2020](https://github.com/JakeWharton/u2020) - a sample app by Jake Wharton that shows how to use Dagger and many other libraries.
* [u2020-mortar](https://github.com/lemonlabs/u2020-mortar) - port of u2020 by lemonlabs that uses Mortar and Flow libraries.