Added Admin Dashboard

This is completed to minimum spec.  

Warnings:
- I'm using rails 2.1.1 so you should update to 2.1.2 in gems

Features Completed
- Active Admin Installed
- Dashboard shows latest books and videos
- Sort books and videos by tag
- Sort most used tags

Challenges:
- Could not authenticate admin (properly) without breaking app.
- Creating Roles without workaround

Needs
- Updated authentication that includes allowing admin to log in to user app and redirect to admin
- Allow roles so that only admin can create new books or videos
- Improve organization of admin interface (optimally have 10 most popular books and videos added on dashboard)
- Allow admin to make user an admin and send email with token to confirm