Add `<MainHeader/> and `<MainNav/>`

* Add `<MainHeader/>` with logo and navigation
* Add `<MainNav/>` with nav items. Also tracks current page
* Update folder structure for /client and /test
* Add `npm run build`
Merge pull request #12 from rtfeldman/add_main_header_and_nav

Add `<MainHeader/> and `<MainNav/>`