Add Custom User Role Support

[JustinSainton]

The current iteration of the plugin's ability to only track certain user roles depends on the "user_level" property of the WP_User object - a property (and concept) that has been deprecated in favor of the roles and capabilities APIs in WordPress.

By using the native and newer APIs, we'll be able to accommodate more fine-grained control here, as well as support custom users and roles that are created by third-party plugins. This would prove especially helpful for eCommerce plugins like WP eCommerce, WooCommerce and Easy Digital Downloads - all of which add custom user roles.