[Radiant] Role Based Access Control extension

[Jim Gay]

Hot on the heels of our Featured Pages extension is an update to RBAC  
Base

SVN repo at http://www.saturnflyer.com/svn/radiantP/rbac_base/trunk

It was mentioned on the list months ago, but hadn't received enough  
attention (internally) to warrant putting it on the http://wiki.radiantcms.org/Thirdparty_Extensions/ 
  but it is there now.

RBAC Base gives you the ability to grow beyond the standard roles in  
Radiant, allowing you to build extensions around your own roles.

It adds a roles table, a roles_users table, and creates the  
has_and_belongs_to_many relationship between users and roles.

* The drag/drop for the users is a little quirky, but it works (and  
will be updated in the future).
* Does not yet provide a way to set the :visibility for your extensions
* Role deletions sit dangerously behind a GET method, so beware, but  
we'll be fixing that too.