Coded CAS authentication

Unfortunately, the CAS client code runs too late in the WordPress
page load process and we're getting an error that HTTP headers (to do
 the redirect) can't be written to because content writing has
 already started. I'm currently troubleshooting this.
Login via CAS is now working

fogbugz:5062
Implemented requireAuthentication as a PHP class, currently using
CAS for SSO authentication. This is a rough, working implementation
that needs polishing, better error handling, etc.
Added enableDebug flag

Primarily for enabling CAS debugging.
Added ability to specify more than one shortcode for requireAuth

When calling the constructor or setup() an array of shortcodes
(strings) can now be specified.
Removed hard-coded values & refactored constants, logging
Merge branch 'dev' into fb5062-ForceCasLogin

Conflicts:
	gravityforms-external-data-fields/EXAMPLE-gravityforms-external-data-fields-config.php
	gravityforms-external-data-fields/gravityforms-external-data-fields.php
Merged in current state of main repo

Including moving a session constant from config.php into a class.
Merge branch 'fb5062-ForceCasLogin' of https://github.com/shawn-bellevuecollege/gravityforms-external-data-fields into shawn-bellevuecollege-fb5062-ForceCasLogin