Updated DJANGO_ROOT and SITE_ROOT to follow book conventions

There are a number of changes I made based on recommendations for the book and my overall experience building web applications on rails and ASP.NET MVC.  They are as follows:

1.  Renamed the DJANGO_ROOT and SITE_ROOT constants to follow conventions in book (p.20)

2.  Renamed the inner "project_name" to "project_name_config" to clarify it's purpose and improve understandability

3.  Upgraded css/ fonts/ js/ to Bootstrp 3.1.1

4.  Updated URL to DJango admin to be 'site-admin' instead of just 'admin'