Make the admin more object-oriented

Move the global variables and functions from 'init.php' into the
'ROOFLib_Admin' class.
Move the class 'ROOFLib_Admin' into its own file

Move the class 'ROOFLib_Admin' into its own file, so admin.php works
similar to roofl.php.  Rename ajax.php and output.php to be more
descriptive.