Gravity Perks - Limit Submissions 1.1.15

Added support for limiting by anonymous users (not-logged-in).
Updated verbiage of "All IPs" and "All Users" to better indicate that these rules apply to "each" user.

Updated role-based rules to use role slug rather than role name.

Updated to use minimum_requirements() method for declaring minimum requirements.
Fixed issue where some time period values could generate notices if not converted to int.