Added an offfical Admin Area

Fixes #1030 

Pretty simple, URLs are:

~/Admin - Home page
~/Admin/Errors - ELMAH
~/Admin/Database - Dynamic Data

Admins get the new Admin tab and can view the Admin home page:

![admin](https://f.cloud.github.com/assets/7574/374656/a4ee4556-a3a3-11e2-8a1d-ca996679a9d4.png)

All Admin things are in Website\Areas\Admin and there's a unit test in Facts\PolicyTests.cs to verify that anything in the NuGetGallery.Areas.Admin.Controllers namespace has Authorize(Roles="Admins")