Closes #50

- './header.php' now utilizes './Account/AccountManager.php'
- './checkout.php' now checks to see if the user is logged in before
writing certain HTML elements to the page
- a link to './cart.php' is now displayed on the nav bar when the user
is a guest
Closes #49

- Just some clean up and tagging
Merge pull request #80 from uptownjimmy/cartnavbarasguest

Cartnavbarasguest