Paid Memberships Pro - Extra Expiration Warning Emails 1.0

* ENHANCEMENT: Updated how the plugin tracks which emails have already been sent. This should fix issues where emails would not be sent out consistently, but may cause users to receive an additional email immediately upon upgrade.
* ENHANCEMENT: Added a new constant `PMPROEEWE_DEBUG` to allow setting an email address to send log data to or to have the log data printed to a file.
* ENHANCEMENT: Now passing the user object to the `pmproeewe_send_reminder_to_user` filter.
* ENHANCEMENT: Added localization support.
* ENHANCEMENT: Now tracking the time that notifications are sent to more accurately send future notifications.
* BUG FIX/ENHANCEMENT: Now compatible with PMPro Multiple Memberships Per User.
* REFACTOR: Now using `get_option()` instead of `pmpro_getOption()`.

* BUG FIX/ENHANCEMENT: If running a current version of PMPro, will use the pmpro_cleanup_memberships_users_table() function before finding expiring emails to avoid certain issues caused by errors in the memberships_users table.